why do so many atheists believe in aliens but not God?
The proof is in the mathematics for alien life, as for God that is faith, as for our existence that is not proof of a biblical God, only proof of a creation of some sort (guess we can call it God, but not a man man book version).
Alien life is nearly mathematically impossible for their not to be in our universe and forget about it if the multi-verse theory holds to be true then it is 100% true that there are aliens.
Is there a God? if you call the creator of existence or point of existence God then Yes 100% there is a God, but not in any way a book or any man could explain.
If atheism means there is an absence of belief that deities exist then the proposition that the arrival of aliens on earth predating theism actually created theism by becoming the original objects of worship, i.e. "Gods" and thereafter the entire construction of religious devotion. In that case it would suggest to me that atheists would be ambivalent about the belief in aliens.
That's actually a very interesting question. To start with, Atheists are more likely to be members of academia, although it might be better to say that members of academia are more likely to be atheists. As such, they would be more familiar with things like the Drake Equation, which is an equation to show the likelihood of communicative life in our universe, and other mathematical and scientific findings towards the possibility of life outside our plant. Extraterrestrial life, to those among the more scientifically mind, is obviously probable. Yes, we've yet to find it, but its more likely that it exists than the contrary, so it's perfectly reasonable to believe it exists.