That's mostly because you're not supposed to be doing that sort of thing in the first place. Customizing widgets = inconsistent look and feel. Custom widgets = Inconsistent LAF, inconsistent interaction, no accessibility etc.
The idea of Win32 UI, WinForms etc. is that you have a look and feel dictated by the system, not by individual app developers, with the ultimate goal being not just a consistent look and feel of the whole system (including applications) but consistently applied patterns of interaction. This in turn makes computers easier to use.
Of course, modern UI frameworks, whether in the browser (JS) or outside the browser (QML, UWP, JS, ...) run completely counter to that.