Well, no it's not just that it doesn't appeal to me, it it scratches a different itch. I just happen not to have that one. Fighting in a tournament to get to special bosses is quite a different itch than fighting a bunch of super bosses.
Do they have to take multiple days to complete? Some people like the familiarity. The change in format is what makes the difference, you're facing them wave after wave and are rewarded with a special boss at the end. Also you can get all kinds of combinations that might not appear on the regular worlds, which can be fun. On top of that you can face off against world heartless bosses again, which isn't possible by just going back to the world. Fighting the special rounds featuring FF or Disney bosses was always the highlight for me in the tournaments, at least in the games that did that.
They're also often good places to grind, if you want to level up to 99. The gates also let you do that, but it wasn't quite the same.
Yes, KH3 had the best version of Olympus, but that doesn't really replace the lack of the tournaments some of us have enjoyed in the past.
Considering how many people would like the tournaments to return, I'd say it's a bit more than just a nitpick.