I get what you are saying, and definitely agree with several people above about the strange proliferation of romance mary sue fics in slasher fandoms, and nobody likes a bad mary sue fic regardless of the fandom.

On the other hand, a lot of people write fanfic to explore a side of a story not already explored in canon. That's why you have WAFF stories written in a fandom where the canon is dark and gritty, and also why you have darkfics written about My Little Pony.