They limit the feature set so they can stabilize on a smaller basis.

They take UI/UX decisions they think are right.

That's what I think.

I also think they managed to build a beautiful UI that works quite well and that pleases many people.

I also think they removed useful features, lagged on important features (thumbnails in the file picker, which I always found bad anyway, including in the Gnome 2 days). I also think they believe they know better than their users on what they need when they really don't, that they should be more understanding of users trying to work around the flaws instead of despising them, and that they shouldn't both reduce the feature set to a minimum and break extensions in each release. And I do indeed think they made UI like it would be used on tablets, degrading the experience desktop, way too early when they didn't work well on tablet anyway for many reasons and KDE was the only bearable option on tablets at the time.

So I really believe they truly do what they think is best, but I also like the KDE approach way better: listening to the users, trying to polish things while not removing too many features, being humble in their decision, and acknowledging their users might have different needs / taste. (for instance, in Plasma 6, they are reverting to double click by default - many KDE devs prefer simple click and think it's objectively better, but they recognize many users are disturbed by this default.)