macOS X was released in 2001 with the "Cocoa" application UI library. While "Carbon" helped older Mac OS applications run for a limited time on OS X, no applications today use any graphics stack older than Cocoa's 2001 release, as the drop of all support for PPC-targeted apps insured that in macOS' switch to x86. (Cocoa had High DPI support baked in from NextStep, even if it took ~11 years to be "needed".)

Win32 was first beta tested by developers in 1992, and has had to remain stable and backwards compatibile to those first early 90s versions. There are still 32-bit apps written in 1992 that are expected to run unmodified on Windows 10 today. The last processor-architecture related API drop that Windows has been allowed by public perception and corporate strategy was Win16 support was dropped on 64-bit processors. (Hence why Windows on ARM has struggled and the current iteration of Windows on ARM now involves a 32-bit x86 emulator as a core feature.)

Mac apps did have to be upgraded, it's just that Apple has been much better at requiring upgrades. There's barely no comparison here. There is no way that you can possibly find today a version of macOS that still supports Mac Classic applications unmodified from the 90s (for instance 94's Glider Pro v1) with High DPI support, yet Windows absolutely must run applications from Windows 95. Sure, Windows sometimes still stumbles in High DPI support for pixel-perfect applications written three decades ago in the 90s, but it at least tries, macOS shrugged and gave up.