Because then it's not a fanfic anymore, it's an original story heavily inspired by another story.
Fanfic is actually pure wish-fulfillment. Have you ever watched or read something and don't like a part of it? Maybe the male lead ends up with a girl you don't like. You're frustrated. What to do? Yep, you write a fanfic where the male lead ends up with the girl you think is more suitable. Heck, you can even make it a harem if you want to.
Lemme give you an actual example, coming from a reader/audience point of view: I remember watching Fairy Tail and thought, "Damn it, when will Natsu and Lucy get together? They clearly have feelings for each other, so please don't do this to us, author!" If you watch Fairy Tail, you'll know the author like to tease us with the ship that will never sail. It's kinda just there at the dock, ready to take off, but... not quite. So what did I do with that frustration? I'm afraid to inform you that I didn't jerk off that day, instead I went to fanfiction and found a story where Natsu and Lucy do end up together. Goddamn were there so many of them! There were some harem as well. Now, that's what I'd call a delightful collection!
Now, if we go by your theory and make something new out of it, that wouldn't be Fairy Tail anymore, it would be something completely new. There would be no Natsu, no Lucy, only something similar. That's fine and all, but it's not quite as satisfying as seeing the actual characters I've come to care about.
Another example I think a lot of people can resonate: ever watched a harem anime and thought, "Goddamn, mc, don't be such a pussy! Being a gentleman is cool, but being a pussy is not! Big difference, my guy. Stop blue balling us with that indecisiveness of yours!"? Now you're frustrated, what to do? You can jerk off, yes, but I would go for a fanfic and search for the same exact story with the same exact characters, but with a different mc. It can be an original character, self-insert, or the exact same one, but their personally definitely has to change. No more blue balls. Now, that's a delightful collection, indeed.