Comment by Crinus

Most modern frameworks provide more functionality than raw Win32, such as automatic layout management and most of those are also cross-platform. People use those frameworks because of those features and/or need to be cross-platform, not because of Win32's ergonomics.

Win32 only provides the basic building blocks for the UI and Microsoft decided that this is enough (i disagree, i'd like them to add new APIs to Win32 for stuff like layout management, resizable dialogs, dockable panels, etc) and anything else should be provided by other libraries, so you just see people using these other libraries.