Why is it that so many people disbelieve God because they cannot see him and then quickly argue that aliens exist dispite never seeing them?

I'm not trying to argue either point here, I'm just curious. Why is it we hold a different requirement for God's existence than for aliens?

Many say that aliens must exist based upon probability in light of unprovable presuppositions. Others argue that God must exist based upon probability in light of unprovable presuppositions.

With that said, many Christians deny aliens while many who argue in favor of aliens are atheists. Why is it so often a person believes one or the other but never accepts both or denies both.

What are the presuppositions that causes this to happen? Why is it that so often people either argue for God or aliens but have no middle ground?