Because I see no reason to believe in God. All the arguments for believing only really make sense if you already do believe. Very few people who grew up atheist look at a religion and go "Wow, that makes so much sense!".
Religions are based on faith, being able to believe in something supernatural despite a lack of evidence. Don't get me wrong, I'm not saying you have to be a naive idiot to be religious ('cus you don't), I'm just saying that the only reason to believe in God is that you do (because all other reasons are only work if you are a believer), and if you don't believe then it's hard to find any reason to start.
Of course, this is just my opinion and it's not based on any research whatsoever.
We have no evidence for one existing and far to often factual errors are found in supposed in fallible holy books, which is the source of these claims.
