This is all completely true but having been in tech through the period I'm not sure it's necessarily the reason. Certainly having flat designs is much, much better technically with vectors everywhere in terms of performance and unnecessary engineering effort, however I those considerations aren't often that influential to designers. I think it was more of a UX cultural shift. When people were moving from physical interaction to flat touch based interaction there was a belief that buttons needed to look like buttons and elements needed to look like physical elements to be perceived. So 3D looks everywhere. Over time the end user understood screen based buttons conceptually without the need for the visual prompts. Or the argument was bullshit in the first place, but companies will listen to UX "experts" and do what they say. And it's a brave company which completely disregards what everyone else is doing. Your design looks different and weird and it seems like you're lazy or don't know what you're supposed to do.

I suspect the intitial flat movement came from startups who couldn't afford the insane effort of 3D buttons everywhere, so didn't bother. The world didn't end and so larger companies learned they didn't need to bother.