That’s a fundamental misunderstanding of what agnosticism and atheism even are. Agnosticism is a belief in knowability, and most atheists are agnostic - they make no claim to have certain knowledge about whether gods exist. They mainly acknowledge there is no reasonable evidence to believe in any gods as presented. Many theists I have known are gnostic, as they claim to have definite knowledge that their god exists. Atheism is just a position on whether gods exist. Gnostic atheism is as dishonest as gnostic theism, but then, I have never met a gnostic atheist. I have met many agnostic theists, which is why they are tolerable to be around. They don’t make any claims to special revelation.