Because reality appears to have a strong atheist bias.
How people become atheists differs from person to person. Some people are never presented with religion as children, and as adults have the critical thinking skills to simply reject it the same way they rejected Santa when they were six. Others were raised religious but as their scientific literacy improved they spotted the contradictions between reality and religious misinterpretations of reality.
Personally I was raised a christian, I quickly learned that god's (or gods') spokesmen on earth were untrustworthy scumbags who sought power and influence over others, and found that religion offered an easy shortcut to gain this power. This wasn't enough to remove my faith, just my reverence of religious preachers. Then I realised that I'm a bisexual man, I'm capable of falling in love with members of either gender and there's nothing I can do about that. This left me with a great deal of cognitive dissonance, after all god is meant to be love right? But if god made me non-straight, and homosexuality is a sin...does that mean god made me purely so he has someone to hate?
I quickly realised this didn't make sense so I became a theist; someone who rejects religions but still believes in a god who cares what people get up to, this allowed me to push past the entire Christian dogma but retain faith.
Whilst at university I decided to read through the bible in full (I was an English Language and Literature student, English literature borrows a lot from the bible) and what I found was that not only does the book come across as less realistic than Harry Potter, but that god, as a concept, is fundamentally totalitarian. To believe in god is to believe that we are being watched and judged every second of every minute of every day. Not just for our actions, but for our thoughts and beliefs. This is literal thoughtcrime, having read 1984 just prior to reading the bible I found too many parallels between god and Big Brother. Except at least Big Brother wouldn't drag you to Room 101 just for having the wrong emotions, but envy is such a harsh sin it's even mentioned in the commandments. It's not some silly little rule like not eating shellfish or wearing two fabrics of clothes or the evil of decorating a tree. (That's right, christmas trees are sinful Jeremiah 10:1-5)
So not only am I an atheist now, but an antitheist, someone who is actively offended by the very concept of god and glad that every religious text is incorrect in their initial assumption, namely the existence of a theistic deity (or deities) excluding the texts of Buddhism and Scientology of course, which are wrong for different reasons. The assumption that theistic dieties exist is not only patently false but would be completely undesirable if true. If christianity was true, I'd ally with Satan in order to dethrone god, as such a throne cannot exist alongside freedom and morality.