Again, it's a collab between Disney and Square. Neither fully own the franchise. Disney tried to make their own TV show based on KH1, which Square put a stop to because they didn't want more than one canon.
The official stance is "because they wanted to respect Mr Nomura's vision", which is just corporate a fluff way to say "yeah we can't do anything with this franchise on our own,"
Yeah, Disney could theoretically buy the rights wholesale from Square, but then they'd be stuck with a property they have no earthly clue what to do with.> and the Final Fantasy characters were honestly never that important and easy to replace with more OCs
You're making it sound like the Disney characters were ever important either. KH is a franchise that runs on novelty to distract how unnecessary and forced the "crossover" actually is.