Much as we might like to believe it, human beings are not rational animals. Imagination plays a very important role in our everyday lives. Impulses like creativity, art, ambition, and purpose all stem from imagination.
Rationality has a limited use in our lives unless our profession demands the use of rational faculties. And even when it does, our sense of awe and wonder often prevail over the strongest evidence. Or rather, in this case, the lack of sufficient evidence in favour of the claim that god exists.
And it doesn’t help that the god concept is fed into people’s minds when they are very young and impressionable. Over a period of years, this idea is reinforced with rituals, superstitions, and the general implication that morality, community, and well-being depend on its acceptance. Add to that the fact that religious indoctrination never really ends and even adults are constantly subjected to it by way of television, books, and the internet.
Smart people are smart and stupid people are stupid. Belief in god is neither indicative of intelligence, nor the lack of it. It’s just one of those things about a person that hangs in their clothes like a smell. It’s just there.