Why not?
Again, how are you going to turn the cash into progress? Hiring someone who doesn't already work on the game would be thorny in many ways, and I don't think there's any serious prospect of raising enough for one of the current devs to quit their job and work on UFO:AI full time (even if you manage to pay their salary for a year, unless they're already unhappy with their job or there's a real prospect of managing to do it again next year that's just not enough—you'd need at least several years up-front to make the risk acceptable).
Mozilla Corporation, in contrast, is large enough to in fact turn donations into full-time employees.
Now, it's possible that there are resources that could be purchased to help develop the game, maybe a dedicated build/test server or something, but there's another can of worms there: I'm pretty sure there's no UFO:AI Corporation, so anything like that would be owned and physically possessed by one of the devs. How do you ensure ongoing availability of that resource? How do you make sure it gets used to develop UFO:AI and not for the personal affairs of the possessor? This can be a problem even without actual wrongdoing—perception can be just as important.
There's really no point in discussing the matter further until we fill in the "???" in "1) Collect donations, 2) Use the money to ???, 3) UFO:AI is better!".
~J