Even aside from original fiction ("original" is always in quotes in my mind because all stories are in conversation with other stories) I have two decades of experience writing fanfic for various spec-fic fandoms, primarily fantasy, so clearly it's not something I'm unfamiliar with. I think it's mostly that when I hear "write a fantasy" or "write science fiction" or so on my brain freezes up with all these preconceptions of what those genres are supposed to be.

When told to simply tell a story, on the other hand, especially with specific prompts that are not genre-specific, I find I often reach for genre elements to tell that story. My brain treats genre as a means and not an ends, so "write X genre" is exactly the wrong approach for me. Rather I'll tell X story and the genre will follow, if any.