I was raised as a Christian, but I consider myself as agnostic for personal reasons.
Anyways, I don't know why the religions are so split up. From my knowledge of other religions, it seems like they're almost identical, but there are just a few similarities. I can only say this for the various branches of Christianity, though.
For the other religions, they all have a central God, they believe that their God loves them, they have certain things that their God doesn't want them to do and they have a certain idea of how their God looks. For Atheists, I guess you could assume that their God would be themselves.
I think religion is the way it is because we, as a race, can't stop arguing about the petty details of it.