I think you'll find atheists in general (though I can't speak for all, because like all groups there will be differences in individual outlook) don't see themselves as denying the existence of God in an absolute sense as you claim, they merely say they don't have enough evidence at this point in time to accept the existence of one, not that their couldn't possibly be one, or that they wouldn't accept the evidence for one if such evidence came to light.
The term "atheist" is a very loose one and is a generalisation for a broad range of ideas that are subtle different. I don't think people who call themselves athiests would limit themselves or define themselves to fit YOUR particular definition.