There are at least five rarities in MtG: Common, uncommon, rare, mythic...and basic land (some sets also have a sixth, special).

Uncommons are silver
Rares are gold
Mythics are vermillion
Specials are purple
Commons are black
Basic lands are...also black.

Sets that have basic lands have at least as many basic lands (generally 4 of each land) as mythics.
Even though there are only 5 cards that are basic lands, you are as likely to get them in a booster pack as you are a rare, and more likely than a mythic.
In premade decks, they occur much more often than rares.

(My vote for basic land expansion color is green).

Rarities have always represented a range of frequencies, not a specific fixed rate of appearance. For example within the rarities you've already identified, there's the 101st common (a common in a large set that appears ~half as often as other commons), there's foil versions, there's the difference between large sets and small sets, and now we've even added Masterpieces (which share the Mythic symbol). None of those get their own symbol. Why should basic lands get their own symbol, when those things don't?

Strictly speaking, the Masterpiece/expeditions do get their own symbol, it's just not differently colored, but it IS a different symbol.

Whether something is a basic land or not is already quite apparent. So it's not like that extra symbol is needed to convey that information. It is true that they don't follow the same booster distribution rules as "other" commons, but that's easy to notice just from opening a few boosters. And I don't think they really want to draw attention to that fact. The basic lands are not supposed to stand out as special (quite the opposite).

I can understand wanting this change from just a "technically, the printing and collating process treats them as their own thing instead of as part of the commons" perspective. But in terms of actual benefit to players, I'm just not seeing it.

And I don't see what's going on with your math. I'll assume a big set and ignore the possibility of foils or masterpieces. You have a 1/5 chance of getting a particular basic land from a booster, or 1/20 if you're looking at different art as different cards. You have about a 1/10 chance of getting a particular common, about a 1/27 chance of getting a particular uncommon, a 1/60.5 chance of getting a particular rare and a 1/121 chance of getting a particular mythic. So they're either more common than commons or less common than commons but still more common than uncommons, depending on how you treat the different art.

While I can appreciate the "basic lands are actually rares" argument, I prefer to appreciate it as a "that's weird and/or interesting" thing rather than grab up a picket to start marching outside of the WotC headquarters demanding a new rarity color for basic lands.

Because as adeyke has already pointed out (though with different words than what I am about to use), you're way more likely to open ten booster packs and see the same basic land type among the basic land slot in those boosters than you are to open those ten boosters and see the same rare in the rare slot.

You're doing a PARTICULAR card.
My math is ANY card of a given rarity.
Each booster pack contains 1 basic land and 1 rare or mythic.
Also, while you have a 1/5 chance of getting a particular TYPE of land, you generally have a 1/20 chance of getting a particular CARD (each land has 4 different printing).

It's not a huge deal, and it might not even be worth changing, but ideally basics should get their own rarity colour.
As for why they don't? Historical inertia I suppose- it's what's responsible for most things.

Er, I already brought up the 1/5 vs. 1/20 thing. Did you not read my post?

I now see that math you're using, but it doesn't make any sense to use that particular metric. Cards aren't considered "common" because there are a lot of them in the pack, but because the combination of how many there are in a pack and how many different ones in the set will mean that each particular one is pretty likely, and you'll start getting duplicates of those earlier and more frequently than the other rarities.

To illustrate why your metric isn't useful, suppose they made a hypothetical set with only a single mythic (and no rares) but 10,000 different commons. That mythic would appear in every booster, so even if it's the most useful card in the set, it would quickly become worthless, as everyone would already have all the copies they need. Meanwhile, the price of the best commons would be astronomical, since there would be so few of them in existence.

Or just suppose that in one set, the whole printing and collating system was exactly the same as normal, but for a single common in the set, they gave it a special expansion symbol color. That card would still be just as likely as any other common, but by your metric, we'd now have a special rarity appearing only about every ten boosters, making it rarer than mythics.

Let me counter by why what a lot of what you've said is kind of missing the point:

To a large extent, this isn't about CARDS, this is about SYMBOLS. This is about establishing a threshold to create a new color of symbol. My argument is that basic lands are widely enough in circulation that they justify a new symbol. The 10,000 commons-one rare argument is a bit of a red herring because I'm arguing that a new color of symbol could be established without tinkering with distribution of cards in a pack. The one-common-with-a-special-symbol is a better argument, and I wholly concede that it would be foolish to print a rarity that only appeared on one card or in 1/10 boosters. Lands appear 10x as often in boosters as the one marked common. Likewise, if there was one card that appeared in every pack, I think it WOULD make sense for it to have its own rarity or symbol.

You still haven't explained an actual benefit to the change. Could there be a special symbol for "either more plentiful than common or less plentiful than common but still more plentiful than uncommon, depending on how you treat different art"? Yes. But why should there be?

Having different colors of expansion symbol in general makes a lot of sense. It helps people interpret the cards, it focuses player excitement, it sets up expectations for what will be in future packs, it helps drafting, it reduces predatory trading, and so on. None of those would really apply to distinguishing basic lands from regular commons, though. What benefit would there be to giving them their own symbol, and what harm is there in letting them keep their common symbol?

I honestly think that, at this point in time, changing the rarity color of basic lands would cause more harm than is worth it. Because you can't go back and change the 23 years worth of basic lands so that they have the new rarity color. So new players who have no idea about older sets facing off against older players might start some argument about how "That basic land is fake! It has the wrong rarity color!"

In 2008, you could have said the same thing about Mythic rare.
In 2006, you could have said the same thing about special.

No. Mythic rares were designed to show up less often than regular rares, which allowed them to be generally more powerful than rares in the past had been. And until Magic 2010, no Mythic Rare that had been printed was a reprint of any other card. Out of 15 mythics, only 6 were new. And 5 of them were really just bringing the original five planeswalkers up to the rarity they had decided was where planeswalker cards should reside at. So 4 reprints that upgraded cards from rare to mythic. And nobody complained about the chosen cards because they all feel pretty mythic.



ONLY ONE SET HAS HAD PURPLE SET SYMBOLS.
And they were all reprints from Magic's past. Ergo, it was a gimmick which they have so far ONLY DONE THE ONE TIME.

You're suggesting a change for all sets moving forward that affects cards that have been in Magic since the very beginning of the game. Mythic did not have any effect on previous cards (except when those cards got reprinted and upgraded to mythic, but so far they haven't really dropped the ball on choosing old cards to make mythic). Timeshifted's purple set symbol has not been reused, and likely never will be.

I don't think the "new players seeing old cards will consider them fakes" argument is a particularly compelling one. There have been various changes to how cards look that are much more significant than the color of basic lands' mana symbol. If a new player first encountered a pre-M15 card, they might think it's fishy; if they see one with a pre-Eighth frame, they might wonder if it's even part of the same game; and if they saw something from the very early days of Magic, they may well think it's just a bad fan creation (both templating and printing technology have advanced a lot since those days). It's easy enough to just go to Gatherer to show them that the cards from back then really did look like that.

The long history of basic lands using the common symbol does have significance, but I'd say it's more about aesthetics than about fakes-suspecting new players.

A design one, not a functionality one.
It's no different than.changing the font or the frame, as adeyke noted.
It has less actual implications than changing the rarity of a card previously printed at common to uncommon, or vice-versa.

Really? I thought they were the same color.

(LOOKS)

Well, I'll be darned. They are clearly different colors.

Barinellos was saying that it isn't differently colored, but that the symbol itself is different (i.e. there's an expansion symbol exclusive to "Zendikar Expeditions" and one exclusive to "Kaladesh Masterpieces").

I don't have access to any recent physical cards, so I can't compare those. But from the images online, the expeditions and masterpieces just look to have the same orange as mythics. The actual cards would also have the foiling that could distort that color.

I've handled many. They're the mythic orange but a little darker oftentimes.

The part about reprints hurts your "this is fake! It has the wrong color!" argument
It's like saying Eldritch Murder is fake because it has a different color symbol than M13 murder.

I was comparing the symbols as the appear in the Oracle text, not on the cards. To my eye looking at, for example Cataclysmic Gearhulk, (all the Gearhulks display the Masterpiece symbol right next to the regular Kaladesh symbol) the shading of the two colors looks different. If they are supposed to be the same then that just reinforces my point: that the colors of the rarities already cover a wide variety of appearance frequencies, so picking out this specific one seems odd.