It's pretty simple,
There is no evidence that God exists. There is no evidence that suggests any of the Gods that humans have believed in is any more likely to be correct than any other.
An atheist, like me, accepts that given the evidence, the likelihood that God exists is incalculably small, and therefore we must conclude that within all likelihood God does not exist.
It's the same reason we discount the belief of invisible pink unicorns. We can't prove they don't exist, we all just accept that given the complete lack of evidence, there is no reason to believe they do exist, and in all likelihood, they do not.