Why does it feel like people who don't want Marvel and SW in KH never really have an actual good reason for why?
From what I've seen, anyway.
My reason (for the MCU anyway) is that it's such a huge franchise with so many different connected parts. It also opens up the door for DISNEY to use whichever newest KH game as an advertising tool for an upcoming film. I would rather world selection be focused on the main plot of the KH game and how the world fits in that game's story.
Even at a huge stretch, all the worlds selected in KH3 worked at some level as part of the main plot. Toy Box and San Fransokyo were used as research for replicas/bringing Xion back (also Toy Box was Verum Rex), Corona and Arendelle introduced new PoH and showed Sora actual death/sacrifice, Monstropolis showed the emotions (screams) being used to bring back Vanitas, Olympus was Sora trying to regain his strength, and The Caribbean focused on death again (and the black box).
I would welcome an animated/comic version of MARVEL in KH. Just as long as it was one character/group for example Spider-Man or X-Men. Honestly, visually it would be way more fun to visit a comic book style MARVEL world than a live action one.
Speaking of visually, I also don't think the MARVEL worlds would really be interesting to visit - since it's always a city. KH succeeds because it's exploring film locations - it's like being in a film. None of the MCU films have locations that would really make me wish I could explore in a KH game. But then that opens up the issue of KH using FILMS and not comics. I think films work best for KH because they are contained stories.
Honestly, the best MCU film that I think can fit into KH is the Sakaar segment of Thor: RAGNAROK. Sora can crash land on the planet, they meet Thor then get captured. You get a Hulk boss fight. Then the plot continues until the revolution happens. A giant heartless appears. Sora, Hulk and Thor kill it. Then Thor escapes to go back to Asgard while Sora goes his own way.