It’s actually very easy to see why and makes a lot of business sense. Look at the movie industry right now, almost anything that’s popular is a sequel, prequel, remake, etc. It’s massively easier to make something based on existing properties. The characters and fans are already there, you just put the pieces together. Right now, theme parks are being run sort of like a movie studio (which imo can be a bad idea as Disneys movie studio has not been the profitable part of the company) Big budgets that have to be reigned in and prioritizing existing IP as it’s simply put, easier.
Think about all that goes into your average attraction. They have an idea, have to match or invent a ride system to fit that idea, then you build the ride, but what if it’s a smash hit? You better atleast have some merch. T shirts maybe, pins definitely, is it worth a new pair of ears, who knows? You order all that ahead of time and… it flops. It’s not loved at all. All that stock sits there.
Same scenario with an IP attached. “hey do we have any movies that people love that aren’t represented well on the parks?” Now with the movie, the ride system is usually far more clear. The story is also easier to come up with. The characters and their personalities are already there. You already know what kind of fun they can get themselves in, heck sometimes you can just adapt a scene straight from the movie! Merchandise? No provlem, we already have a ton of Moana, Zootopia, etc merch. Doesn’t sell at the parks? Doesn’t matter, people will buy that stuff outside the parks still. Artists already know how to draw Hei-Hei or Judy fairly easily. Plus now you have a reason for people to actually plan a visit.
Black hole rollercoaster in Epcot? Eh sure, that’s cool or whatever
Guardians Coaster? Oh my gosh that’s favorite ride
Now you can dip into the “worlds” of that Ip. Meet and greets! Food! Specialty merch! It’s all there and not something most non IP rides can get