Because actual fanfics are done by people that love the source material and wish to remain true to it.
Expanded works are done by people who want to: make changes for the sake of their own “artistic” originality, do what they think will make people watch it (doesn’t mean it has to be good), which are both tied to studio execs all wanting to push their pet version of whatever the product will be based on budget or ego.
Because actual fanfics are done by people that love the source material and wish to remain true to it.
Expanded works are done by people who want to: make changes for the sake of their own “artistic” originality, do what they think will make people watch it (doesn’t mean it has to be good), which are both tied to studio execs all wanting to push their pet version of whatever the product will be based on budget or ego.