Because we know that people are flawed, are prone to lying, have limited senses and limited brains that interpret those senses, fallible memories, susceptible to hallucinations and can be insane.
I don't want to be accused of proselytising the religion of science or anything, but there's a good reason that the scientific method relies on independent verification, repeatability and, wherever possible, the lack of reliance on human senses and perception. It's the best way we have for controlling for those human flaws, and it's rather telling that no religious or supernatural claims have ever passed that sort of scrutiny.
As for the OP - Atheism is simply the lack of belief in any God or Gods. Many atheists may well go further than that and have a positive belief in the lack of a God or Gods, but that's not what atheism is. And even if it were, that's not a belief system.