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Because people are God wired from birth to believe in God until indoctrinated out of it. I've found most people tend to convert to atheism in their adolescents before their brain and philosophy manages to develop. So a point to highlight is one takes a philosophically perspective more seriously.
I could equally ask a more valid question of how anyone can be atheist rationally. If atheism is true, it undermines reason and logic. Reason relies on free will, a rationally and objectively free possibility. Something that our studies on evolution tends to contradict. As Dawkins puts it "you're nothing more than a random collection of molecules dancing to the tune of your DNA". We have no rational basis to believe in reason, scientism (know the difference from science) and science tends to contradict naturalism so atheism by it's very own worldview doesn't make sense1