Simply put, you are under the false assumption that evidence is a two way street when it comes to existence. Evidence is purely a one way street because only existence can leave evidence to share. So there is either evidence for existence or there is not. When dealing with possibilities, there is still an obligation to demonstrate or reason a "real" possibility to declare it "potentially" undiscovered. For example, life on this planet makes it clear that life is in fact possible in this universe, so we can be "rationally" agnostic of other life because it has been demonstrated to be in the realm of "real" possibilities. Imagine doing this with magical fairies. Also your assumption is that Gods cannot be disproven and this is false. Gods can be disproven in a similar fashion as square circles can be disproven. You only need an impossible contradiction to "know" they cannot exist. With the concept of Gods, you could write a book on impossible contradictions to know exactly why Gods are purely man made. Putting that aside, the reasonable, rational default position has to always be nonexistence until such time as evidence or reasoning can determine a "real" possibility to be agnostic about existence and much more evidence to support actual existence. There is no reliance on science here other than having an understanding of nature, anything is open to using, including human psychology and an understanding of what irrationality and poor reasoning looks like. Without these reasonable rules, than in your world, anything and everything is possible if you can "imagine" it. That is the sole purpose for the rules, to prevent irrationality where if you are going to "claim" existence, or "potential" existence, you have a burden to show your work on how "you" determined this real possibility or actual existence. Your failure to understand the pitfalls of human psychology and that your "thoughts" produce "real feelings" even if the thoughts are of an imagined concept. That is why we love fiction, it produces real feelings that captivate us. God believers never make that connection and use their very "real" feelings to assert the thing they are thinking of (God) is real. There are rules of nature that cannot be violated, which is why an understanding of certain scientific fields allows you to "know" what these rules are and why they are. It the reason that anyone who understands the definition of a square and a definition of a circle can "know" why a square circle is impossible. If you lack knowledge of squares and circles, you would not know why square circles are impossible. God believers often lack knowledge and often distort real science to fit their delusion. There is other knowledge besides knowledge of science and all knowledge is valid as long as you can demonstrate the "claimed" knowledge is actual knowledge and not just opinion stated as knowledge.