> it's just their own fantasy worlds are even less unique than ours

They do have many more fantasy worlds though, not being as unique means they have room for many many more and better ones. I do feel western authors focuses too much on being different so often they make things bad just to not be like the others.

Do copyright rules exist differently in Japan than the US? Can you imagine if I created a movie that occurs within the accepted Star Wars universe so that my protagonist was from a planet called Tatooine who had a sister that was a princess? I would be sued into oblivion.

Of course you do the copyright scrubbing. Tatooine/Force/Jedi becomes Batoone/Energy/Cyberist(or something similarly inexplicable).

They're stricter in Japan if anything, but companies are better at knowing where their creativity comes from. (Domestically, I mean. They're pretty bad at it internationally.)