Modern UI design is optimized for screenshots in a PowerPoint, and from there, to screenshots in a portfolio and on marketing pages where the audience that needs to be satisfied is marketing themselves, and especially people very concerned that every piece of everything be "on brand" and who are entirely sure this matters a ton for making sales.
That's what's going on, as far as I can tell.
It is like a mass delusion or something that some segment of the population seems to think it that flat design is actually useful for the everyday user.
We've seen other stupid useless crap spread like a wave across our industry, across our society... so I guess I shouldn't be surprised. Though I've started to question my own sanity.