...and the reason this happens is capitalism.
Really, I think there are a lot of reasons why things have gotten flatter over time. A lot of times this isn't exactly what I'd call UX design per se (which herein I define as 'the process by which we center the needs of people when we design things). A lot of it is really trend-driven, yeah.
But if we dive a level deeper, I see this coming back to profit-motive in some really frustrating ways.
The OP actually makes a motion in this direction talking about how some of it is due to covering phones. I'd go more broadly: These design trends, and reliance on design systems, is in part done in the interest of making it easy to scale. It's never enough to build an app that does one good thing on one platform. You have to be ready to do everything on every platform. You're not just a chat client for desktops, you're a "communication platform" across desktop, mobile, and gods know what else.
This also ties into something I've lamented myself, which is how much apps have moved away from just having "fun" features. This really hit me the other day when I was playing around with the music app on my old 3DS, discovering it had not only all sorts of little Nintendo-themed visualizations (in 3D!) but also some audio filters which, while being objectively bad, are still silly fun. And I was thinking, when was the last time I got excited about the visualizer on a music app? My phone apps don't even HAVE them.
So here's the thing. Every feature that gets added to a commodity app has to be tied somehow to metrics, all of which ultimately come down to "making money," and fun doesn't make money, at least not in a way that makes it work out for the people who make product decisions. You might not believe it, but 'designing for delight' is a thing UX designers actually talk about a lot as somthing to strive for. To reference my definition of UX design above, it addresses a user need, in this case an emotional one. Fun feels good. But tying that to revenue is very hard, and so that stuff gets cut, or never prioritized in the first place.
Aesthetics are, in this perspective, a form of 'delight', the one that actually get some attention, because your app has to look like something. But there's the cheap-as-free way to just 'put some aesthetics on it' by using that trendy minimalist flat design. Because doing more requires more effort, for unclear profit, so doesn't get prioritized.
Yes, love this breakdown. Hard agree.