Probably because of potential concerns with moisture/mold build up, wind buffeting that wears the paint, covering a dirty camper, etc...
Good luck,
it's one of those personal preference items as it seems as many as like covers, just as many don't - Just as covers have improved, so have rv finishes such that they are better in the raw
obviously a permanent rv port that doesn't need to be replaced every few years is a better option imho... ours is stored under a storage lot's permanent cover so I don't have to make that decision and am glad, because wrestling a couple hundred pound cover up and over that high an object is not my idea of a fun and safe activity
Full timers don't use covers...