...and it's not an assumption, nor is it evidence. Saying that it "could easily be an illusion" is not assuming anything, it's just stating that "you cannot be sure there aren't other factors involved, for good reasons".
The phase "it could be an optical illusion" is not science. There could also be "invisible fairies" in my computer monitor, but saying it does not make or imply that it is so. Phrases like "you're experiencing a massive illusion" with no further evidence is pure pseudoscience and reflects the absurdity of the Round Earth model.
Then why do you assume that looking out your window is evidence for a FE?
I don't assume that I'm experiencing a "massive optical illusion" when I look out my window. You do. More pseudoscience.
Its not an assumption. If there is a possibility that what you're seeing is an illusion, then there's no reason to believe that what you're seeing is real.
There's also a possibility that "god did it" and that "ghosts exist". Possibilities are not evidence. By telling us that "an illusion did it" with no further evidence you are committing a crime against reason. "An illusion did it" is neither evidence or an explanation. It cannot be considered as a serious possibility unless there is actual evidence which demonstrates it to be true.
This is why FET is a solid science and why RET is a bunk science. FET has us proceeding only by inquiry, experiencing and testing for our own selves. RET has us imagining "illusions" everywhere we look.