Apple's skeuomorphic phase overlapped the Retina display era, though, so I don't buy that explanation. Also, it's nothing to do with raster vs. vector. The photos that we take with phone cameras are raster graphics, for example. They look great, and it's because they're high-resolution. High-res raster UI elements would look great, except then the versatile manipulation by CSS would not be possible. Vector graphics are very good at that.

But here's the thing: Complex vector graphics exist, too. There were some pretty fancy PostScript graphics even back in the early 1990's. With all the pixels that we have now, we could have good design instead of flat, if the developers bothered. But it seems we've internalized the feeling that we're not worth the effort, aesthetics and color aren't interesting, and life is a joyless slog. Which sounds and awful lot like clinical depression...

(Incidentally, odd that emoji aren't flat design.)