Well, I believe one very likely explanation is simply: genetics.
Some humans simply have a brain structure that is conducive to the belief in God, and intense feelings of spirituality. Some refer to this as the "God spot" within the brain.
Those who do not have the genetics for a strongly coded "God spot" do not believe, while those who do have the genetics, have a much stronger likelihood for believing in God, or exploring deeply spiritual ideas.
This could even be a drive towards other forms of spirituality such as poetry, and meditation.
ALSO... Assuming the "God spot" genetically based theory is true, if you are one of those people who is strongly coded for a "God spot" neural network in your brain, then it may be troubling for you to find out that your strong emotional belief in something supernatural and greater, or a yearning for something deeply spiritual/poetic, is genetically coded for.
In other words, you might feel like you are simply a "spiritual-machine"!
But other emotions such as love, desire, passion, curiosity about the world, etc... are also based in evolutionary genetics and that doesn't make those other emotions any less real or less legitimate.
And of course... I suppose one could also argue that if God does indeed exist, then God would naturally want to introduce some genetic programming into at least some of his creatures, for heightened spirituality.
Another strong reason that I suspect some humans come to believe in God, is much darker:
a justification for psychopathic tendencies.
Some people, unfortunately, simply want to hurt, harm, torture, kill, rape, and just generally spread misery to others in their life.
Those people are also strongly attracted to religious ideology, as a way of justifying their urges.
If they can reason that they are killing in the name of God, then it becomes easier for them to justify their actions without worrying about any faint moral inhibitions they may feel.
This type of belief in God, is explained by our ancient ancestors (the ones who were spiritual but not psychopaths) as essentially being "devil" or "demonic" worship, and a belief in false-Gods. This type of belief in God is essentially the essence of "evil".
In that sense evil is very real: it is a genetically based psycopathic drive to hurt others. And as our ancient ancestors explained through their mythology, such "evil" has the power to inflict great suffering upon innocent victims.
FINALLY... one other possibility for people's belief in God might be because God actually does exist.
The universe is proving to be far more mysterious than we suspect, so I do not fully discount this possibility, yet.
There is afterall the paradoxical issue, in which it really does seem like it is "turtles all the way down".
By that I mean this:
Q: How did humans come to be?
A: An evolutionary process on Earth, that began 4.5 billion years ago, with the first single celled life.
Q: How did Earth come to be?
A: A process that began with the big bang, that gave rise to a universe with the physics necessary for stellar and planetary evolution, and organic chemistry.
Q: What gave rise to the big bang?
A: We're not sure, but perhaps it was a result of fluctuations in the multi-verse.
Q: What gave rise to the multi-verse?
A: We're not sure, but perhaps the multi-verse came into being, due to quantum fluctuations, in which something can appear within nothing, but only if it eventually disappears in the future, to balance the equations back to zero.
Q: What gave rise to a nothingness, capable of experiencing quantum fluctuations, that must then vanish to re-balance themselves to zero?
A: Umm... turtles?
In other words, thus far, we can keep finding answers and new plausible/rational theories to explain one macro-layer of the universe, and the multi-verse, and beyond...
But eventually this will become unsatisfactory, because it will be seemingly never-ending, going on for infinity, in which one thing explains the next thing, which explains the next thing... endlessly.
All of which seems might eventually begin to hint at a guiding intelligence... maybe.
But then, of course, if there is a God and greater guiding intelligence to the multi-vast-cosmos... then the question arises:
What gave rise to God?