Why do you believe in an immaterial nonphysical God-being, which can't be observed through scientific investigation, but you dismiss the idea of (mostly) immaterial, (mostly) nonphysical nature spirits called fairies which can't be observed through scientific investigation?
my understanding of the psychology behind
belief is that its not merely enough to make an agnostic conclusion that "it could be", but rather a positive conclusion that "it must be", that it is somehow necessary for the belief to be true.
for that to happen, it has to explain something that doesn't make sense to that person otherwise, it has to delve into what the believer would experience as the great unknown, and somehow resolve it.
fairies represent a mental set where the realm of the unknown was much intimate, where the growth patterns in nature didn't make much sense, where people dead in the forest didn't make much sense, where natural abortions didn't make much sense (though that was usually elves).
theism pushed the line much further - existence itself, mortality, morality, etc'. making sense of questions that for many still don't make much sense, still in the realm of the unknown. its that very intrinsic minimalism in the essence of what made it survive so long and flourish even under sociological stress.
this is why - while both might gain the same agnostic approval of "they can be" - theism maintains the strength of the positive psychological approval that the belief in fairies has since lost.