The same reasons that you don't believe in Islam/Hinduism/Christianity etc - minus the one you do believe in, obviously.
I'm doing fine without either of them.
Because God is just an elaborate version of an imaginary friend that 90% of the world's population seem to share.
No FAITH............... in the word of the bible and all other evidence.
Logic and rationalism. Everybody on this planet practices some form of selective atheism anyway.
Same as above. No evidence and it seems a bit ridiculous to believe in a 'god' who made us and the world. I know people who do believe say that it is a sad existance if you have nothing to believe in but that's all it is, a belief, no way of knowing you're right or that there is another world waiting for us depending on how we behave in this life... Sorry, I'm just very cynical-minded about how this all came about and how intelligent people can have totally blind belief. Seems like a game to me - do well in level 1 and you'll get extra bonuses in the next round!
And the bible as proof? Er, that was written by the supposed jesus' friends, 30 years or so after he'd died (I think so anyway). It's only their word that's followed for 1000's of years!
Because rationalism dictates you don't place favour on unjustified notions. It's inherently unjustifiable, as it claims to know things that are wholly beyond the realm of possible experience, hence by definition one can not know such things. It's very much mirrored in Bertrand Russel's Teapot analogy, highlighting the nature of believing in unfalisifiable notions.