I'm guessing its there to make the effects of a Vehicle more consistent and predictable. If a Vehicle has 5 power and crew 3, that means it has the potential of adding 2 more damage to your attack. If it was based on toughness, it would instead be anything from 0 to 5.
It's a very abstract, gameplay-driven mechanic, so a flavor-based argument doesn't have much weight.
I think the vehicle gimmick is too consistent for its own good. It feels like a choice between running
Trumpet Blast and
Fleetwheel Cruiser, in which you'd normally pick the group buff instead.
If it cared about toughness instead, then obviously the cost of the vehicle would go down being that it forces players to use something gimmicky and with obvious Timmy style, but gives an even greater reward for those that do follow the card's plan: play creatures that you'd normally never play in order to get a fatty into the field to cause damage.
It would be weird though to have Donald Trump's
Wall to suddenly drive an
Irontread Crusher to get to
7/11, but that same wall can drive a
Renegade Freighter.