Why is fanon seen as bad in fandom spaces to so many people?

Over the years on different accounts, I have seen so much shitting on anything fanon. Fanon ships.(Straight or queer) Any headcanon or fanon trait added to a character. Fanfiction. Etc. Fanon is seen as you LARPING to a lot of people and idk why.

Idk if it's the majority (hoping and thinking it's not, if it is well fuck) but it's definitely not an uncommon thing. Shipping is especially bad. Like, even if they don't have chemistry in the show, these ships are still interesting to explore because of different relationship dynamics that could happen, you can play around a lot with ships. Character traits that are fanon are fun to add, or if it's a queer headcanon, it's usually queer people heavily relating to a character but relating to aspects that relate to their own queer journey (coming out, transitioning, bigotry, etc)