Because of Santa Claus.
Yes, this is going to take a bit of explaining. I was raised by parents who encouraged creativity and curiosity, who had shelves upon shelves of books on all walls of the house. And I had access to books as well: science fiction, fantasy, nonfiction. I remember quite clearly reading and rereading a science encyclopedia aimed at young children. I remember regular trips to the museum in Denver. I remember learning about how science worked by observing the world, and rejecting a belief if what was observed did not match the belief.
And I remember, at age eight, applying that method to two hypotheses: God and Santa Claus.
One hypothesis seemed to make sense (remember my age at the time). Every year, at the same time, gifts would arrive overnight addressed from a person named “Santa”. In other words, the observation exactly matched what the belief predicted. The only way that belief could be wrong was if there was some manner of deliberate deception perpetrated by my own parents, and that was (at the time, remember) unthinkable.
The other hypothesis made no sense whatsoever. I had never seen a god. I had never seen a miracle, or any physical proof that suggested a miracle had ever happened. I had never spoken to anyone who claimed to have seen a miracle firsthand. All I had to back up that hypothesis was a copy of The Adventures of Jesus And His Disciples and various stories I had been told in preschool, and at that age I was well aware of the concept of fiction.
And so, one was retained and the other was rejected.
(Of course, I’m leaving out the bit where before that, I read the newspaper and independently stumbled across the Problem of Evil, because I still believed [kind of] after that and it doesn’t make quite as good a story anyway.)
It was not until later that I realized that religion had caused any harm beyond people believing in fairy tales, or even that I was not alone in rejecting that particular set of fairy tales. Only later did I find out that there was an option beyond xtianity, or that others had taken it. And only later did I see a reason to mention my atheism in mixed company.