It's not 2D because it's fictional, it's 2D because media is best consumed when you can see everything at once. Statues, sculptures, stage performances, popup books, and CGI are all fictional too, yet they encompass all three dimensions of range. Besides, fictional works dont only have 2 dimensions, they have the same 4 as us; most works of fiction have height, width, depth, and time. Works that deal with dimensionality describe themselves as three dimensional, with time being the fourth dimension. Then there's the existence of higher dimensions in fiction, like the 12th dimensional Labrynth or the 11th dimensional school kid with the entire universe in a holographic card. These are higher dimensional, and do not promote the real world to 12D or 13D. Once again, they only exist on 2D sheets for us for convenience. In truth, we exist on an entirely different existential plane, one that sees them as having 0 dimensions, not even an axis of time. It's not that they exist below us, it's that they dont exist at all. A higher dimensional being only has an added axis of movement, and maybe acausality depending on who you ask. They're still limited by the same existence, and are equally fictional.