Because the claims that most theists make about their gods are patently incompatible with ordinary facts that anyone can observe about the world. Gods who involve themselves with historical events could be expected to leave traces, just like humans who do; but there are no historical traces of any gods. Gods who get involved in biology could be expected to leave traces in biological organisms; but they don’t. Gods who perform daily activities in response to prayers could be expected to sign or otherwise designate their work; but they don’t.
Atheism is not about claiming there is nothing in the universe that might —if we discovered it—be described as ‘a god’. It’s about denying the claims made by theists that supernatural beings are routinely mucking about with human lives and human history, without—for some unexplained reason—leaving any objectively identifiable traces whatsoever.
Even though they allegedly want us to believe in them.
Jon's Quora content 