The difference is that the vast majority of third party app devs developing for macOS use Cocoa/AppKit, the system's native UI toolkit. Because of that, all App

The difference is that the vast majority of third party app devs developing for macOS use Cocoa/AppKit, the system's native UI toolkit. Because of that, all Apple had to do is get HiDPI mode and a decent fallback (2x scaling) working in Cocoa and boom, 90%+ of all Mac apps, even those that stopped getting updates, have proper retina support. Devs didn't even need to recompile; at first it was an option in a property file in the app bundle (enabling users to "turn on" HiDPI support in older apps by editing the file), and later it was turned on for all apps. Compare this to the situation on Windows where people using the native toolkit is a rarity, with tens or hundreds of different UI toolkits in use and even a bunch of totally custom UI, much of which assumes a 90 DPI desktop. There's really not much MS can do about that except make an educated guess about offending programs and try to force-scale them to 2x.