Aliens, at least highly advanced ones, actually function very similar to angels, gods, demons, etc philosophically. Imagine an alien species that evolved a hundred million years before we did. Unless we’re already close to some upper limit to science and technology, such aliens would be as gods to us.

Alien visitors thus end up functioning very similarly, philosophically speaking, to gods and angels. Traditionally unknown natural phenomena would be attributed to gods. But aliens can serve the same function. See something strange in the sky? No matter how strange, super advanced aliens might be responsible. We have no idea what they’re capable of. Astronomers discover a star behaving really oddly? Could be aliens. Strange comet flying through the solar system? Could be an alien space ship.

We cannot begin to predict the capabilities of a species millions of years more advanced than ourselves. Thus, just like the gods of old, advanced aliens can be used to explain any unknown natural phenomena. Instead of god of the gaps, you get aliens of the gaps.