Flat design is basically a form of elitism. In order to understand how and why the computer works, you must subscribe to a cathedral of thought that exists at this stage more or less to protect itself and celebrate its own brilliance, because any real invention in the UI space had already basically topped out by the late 90s.

From this perspective, we have flat UI for essentially the same reason we're starting to see the formation of tech unions. It has been a long time since a lot of our industry has done anything new or even productive, and much of what we do is entirely self-serving, creating a kind of fragility that ultimately threatens the workforce. Endless UI churn producing worse and worse designs isn't a problem, it's a symptom that is impossible to ignore.