That might make some sense for the web, but doesn't explain it for iOS and macOS, etc.

Apple started pushing flat design just as we were getting high-resolution retina displays for phones. Since the UI art wasn't defined with a vector format, the minimalist icons for flat design was still being provided in big multi-resolution .pngs, so there was little or no savings on resources.

So those UI image files could just as easily contain detailed shaded and/or textured images that make the most of a high-res color display.