Most Zombie and Vampire creatures in MtG are human, and there's nobody that shares both the Human and Zombie/Vampire subtype. Or both the Zombie and Vampire subtype for that matter.

Meanwhile, having other subtypes become Zombies isn't as difficult. There's Zombie Elves, and most vampires and zombies came from humans.

I mean I guess for gameplay purposes, the distinction is needed for expansions like Innistrad and the like. But still, what reason makes humans lose their ability to synergize with other human cards once they become undead?

I guess that kind of matter could be used for a set.

There's no real flavor reason. That said, when people think of undead, they think of human undead first, so maybe we're a "default"

The reverse is also true - when people think "human", they think a living, flesh and blood person.

I think a miserable pile of secrets