...but, as others have noted, even if you want to only support the "core" features that most people would take for granted for a GUI (cross-platform, all the "usual" widgets including complicated ones such as RichEdit, ListView, TreeView, text with Unicode support and subpixel rendering etc. etc.) it's already a massive undertaking.

That is another issue, yes, it is not trivial (though it isn't like making a browser or even a game engine - especially since game engines nowadays tend to have their own GUI toolkits :-P) but my point was that you do not need to replicate what Qt does to have a useful and usable library.