Any number of reasons, but the one I think is most prevalent is that as atheists, we are a living embodiment of a completely alien notion of existence. Most of these theistic people live their lives under and around the commonly held belief that a god exists, and so when presented with someone who doesn't believe in any, even a different one, it causes some confusion, which is where a lot of the misapprehensions about us come from. Like us being devil worshipers, for instance, which originally was attributed to any and all non-christians, by christians.
In short, it's because we exist. We lack a belief in something that they're of the opinion makes them special, and us denying that something lessens their opinions of themselves.
In short, it's because we exist. We lack a belief in something that they're of the opinion makes them special, and us denying that something lessens their opinions of themselves.
If you believe it, question it. If you question it, get an answer. If you have an answer, does that answer satisfy reality? Does it satisfy you? Probably not. For no one else will agree with you, not really.