It only uses the world and the names of the characters because the characters have a different personality and make decisions that they would not make...
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LIKEa fanfic is using the characters from anime series etc. but at least you respect the background personality unless you change them but you respect the original but you can't make a personality so different taking decisions that the original would not take in a fanfic it would be the same as using the name and design of the character but doing it with a personality of a oc.
I do not think I had specified what I meant lol