Why would theists be the only ones who get to define "god?"The word "god" is loaded. I think that there are cases where we can say that a specific concept of "god doesn't exist. So for instance, if someone says that "god" in their religion did something, and we can prove for a fact that that action did not happen, then we can say that their concept of "god" doesn't exist. Alternatively, if I say that my appetite is my "god", then that concept of "god" definitely exists. But overall, "god" is too broad a term which can be applied to different concepts to say that "god" doesn't exist. There must be a context in which to critique it in order to do so.
If *I* don't think that your appetite would qualify as a god, then I wouldn't need to take it into account when deciding to say that there no gods.
Arguments over the existence of gods are a bit unique in that they often aren't just a matter of empirical facts and ontology (i.e. do we have reason say that the thing being called a god exists?). They also often come down to semantics (i.e. can the thing they're calling a god rightly be called a god at all?).
The question of what makes a god a god has become more important over the years as theists have redefined "god" from the more traditional ideas of godhood to things like "love," "the universe," or "whatever is the most important thing in your life."