There are actually a surprising number of people who believe in extraterrestrials, ghosts, and other paranormal phenomena and yet do not believe in God. Even still, there are also a great deal of people who believe in the afterlife, in spirits, and yet do not believe in God. This is, no doubt, a cherry picking of intuitive beliefs.
The fact is that the mind does not form purely original thoughts. It only synthesizes and forms relationships between perceptions.
So, while the Santa Clause who we know and love from his conception as a marketing scheme by Coka Cola isn't real, he was still synthesized from the real Saint Nick.
And while U.F.O.s may not exist within the folklore of little green men, they were still synthesized from the more vague sightings, or at least a common Tupperware saucer.
Centuries ago, when Europeans first discovered the Americas, the natives were so astounded by the ships and sales because it was such a potent synthesis being perceived all at once. So potent, in fact, that the natives literally had trouble seeing the new technology, and that they dubbed the newcomers as gods.
Now, the old pantheons of Greek, Roman, and Gaelic gods, followed by the more animistic spirits of totemic cultures, certainly seem to be synthesized as well, either as anthropomorphized and human-like or as spirit animals. But the newer, more metaphysical notion of God is so vague and featureless that it's lacking in synthesis. In fact, you might call it the foundation of synthesis.
Everything came from somewhere, just know that.