I mean there are a lot of reasons, but the main one is that the anime industry has its shit a lot better than the Western equivalent and the night novel->manga->anime pipeline.

First, you have sheer quantity and variety. Every season, meaning four times a year, more than 50 anime are released, in all genres and for all ages. Meanwhile almost all Western animation is either for kids or marvel. Compare the darkest or most brutal Western animated show you can find to stuff like Perfect Blue (huge trigger warning BTW) and Made in Abyss. I, personally, watched more than a thousand hours of anime, likely more than all Western animation not aimed at kids below 10 put together. It just doesn't begin to compare. Even popular titles like Adventure Time or Gumball are for kids; they're just high quality works that also appeal to adults (more on this later). I know series like Invincible exist, but seriously. Name me 10 of them. Anime is a huge industry of its own right, more comparable to Hollywood than Western animation, and I think we all know Hollywood isn't interested in making anything decent right now. That's part of why anime is so popular.

Second, with anime there's usually a story that's being adapted. This means there's a lot less of the hit or miss aspect surrounding a new work, as a manga or novel needs to have a certain amount of quality before it even qualifies to be made into an anime. Also, the market cares more about its customers than in the West, so studios do their work more faithfully (otherwise they won't get new jobs). As someone making a new anime, you want to sell blue rays, you want people to buy the original work, you want them to buy merchandise, and for all these you need to create something good that will actually turn in a profit. Also, if they've got a good anime going they don't suddenly decide to kill it and spend the money on another yacht. I'm still salty we didn't get a proper season 3 for The Owl House, for example. Studios have more respect for the work they're doing, and an original story they have to follow or they won't be getting any more work. Nothing like the MBA infested mess that exists in the West.

Third, anime and manga aren't tied to a certain age in Japanese culture. 60% of Japanese people watch anime at least once in a while, and a similar percentage reads manga. It's not something you graduate as soon as you enter middle school. Light Novels are also obviously not for kids, because what kind of kid reads for entertainment today? These media all lean towards teenagers and young adults, and generally don't make too many assumptions about the viewer. I mentioned Adventure Time up there; so even anime that's made for kids doesn't treat its viewer as an idiot, which makes it watchable as an adult. Now how watchable depends on your tastes, but even a straight shonen like My Hero Academia or Demon Slayer has reasonably realistic characters with personalities, as tropey as they may be. Even the most shonen of shonen anime passes the same standard that makes Adventure Time and Gumball watchable to an adult compared to something like Paw Patrol.

So, yeah, it's not even a comparison at this point.