I mean, that's debatable. But in any case Xlib, Xaw, and Motif can be completely bypassed nowadays--XCB and Wayland are complete replacements for Xlib (with EGL finally providing the needed XCB-compatible alternative to GLX), and GTK+/Qt are alternatives to Xaw and Motif--while Win32 is still the only way to do many things on Windows. Granted, the vast majority of devs use wrappers around Win32, so they don't have to care about the low-level plumbing.

Which have very sparse documentation, but do work just fine.

Win32 covers a lot more than xlib, athena and motif. And still I have to resort to porting over bits and pieces from the FreeBSD C library because in a pure win32 project (no msvcrt), things are missing.

The thing is, getting all the details right requires extensive research and a lot of extra nonsense that shouldn't be necessary. For compatibility, accessibility, consistency, etc. Sometimes it's amazing things work at all.