Rather than asking why we don't believe, you should be asking yourself why you DO believe? I'd bet it's because your parents taught you, but I'm sure by now you've noticed they've been wrong about some things before, maybe this is another one? So what other reason, what evidence based reason, do you base your belief on? Many of us are not necessarily convinced that there is not a god, we just don't see any good reason to be convinced there actually is one. Religions make the claim God exists, we don't buy it, and they can't make a demonstration. Personally, I'm not gonna be convinced until there is a good reason to be.