I think one reason is that flat UI is super easy. Skeuomorphic is extremely hard to get right, and if you don't get it right it looks super tacky. Most people who have the word "designer" in their job title don't have the artistic skills needed to pull it off. This is why most designers are opposed to skeuomorphic.
Somewhere in between is the right approach. The NeXTSTEP UI from the late 1980s is what we need to return to. It still looks beautiful today: https://guidebookgallery.org/screenshots/openstep42