Designers tend to be less open to feedback than developers. That, I think, helps explain why flat UI persists even though it has shown usability drawbacks. It also helps explain why overall usability feels like it's declining ever year — for instance, macOS Tahoe seems noticeably worse in usability compared to macOS Sequoia. Does anyone think Apple is going to rush out a release that fixes the excessive rounding of w…
On the topic of flat design specifically, developers are likely just as culpable. Back when it was just starting to catch on, by my observation some of the quickest to adopt it were solo developers because it's way easier to build a passable looking app with flat UI since that doesn't require any design talent.
Unless something like https://kde.org/announcements/plasma/5/5.12.0/spectacle-noti... is what you consider to be passable looking of course.