There is no reason to think that a god exists. If you hadn't been brought up in a religion, then you probably won't believe in one. There is no evidence for a God or supernatural being, so why should you believe in one? Most people don't believe in unicorns, fairies, and the giant spaghetti monster in the sky. Why? Because there is no reason to believe in one, no evidence for it. There are 1000's of religions, how can you be certain that one is truer then the other? They can't all be true. All have the same credibility as each other.
As science pushes onward in its understanding of the universe, the room for God shrinks. At the time the bible was written, the known universe was the middle east, Europe and africa, as well as the heavens. It seemed fitting then, that a god would create it all for man, as it wasn't all that large.
Now look at it. We have the Hubble Ultra Deep Field image, showing 1000's of galaxies, billions of lightyears away. Each consisting of 100's of billions of stars. And this was only one tiny dark spot in the night sky. The size of the known universe is now ~96billion Ly's in diameter. It doesn't seem fitting that a God would create all this for us, It doesn't make sense.
Maybe, a few thousand years ago, a God was comforting. It gave you a purpose. It could 'explain' the explainable of the time. Now, science is uncovering the secrets, it has answers to the things that were once 'explained' by religion.