that's because the native widgets haven't changed much
If browser makers can't maintain something as simple as a radio button, they will probably not be able to maintain more complex widgets.
Browser engines are already so complex that even a giant like Microsoft gave up on maintining one. And Firefox is spending over $200m in software development per year to keep up.
Who said they "can't maintain it"? They just wont, which is different. Besides, it is up to HTMLWG/W3C to expand on those widgets, and they have done nothing...
>Browser engines are already so complex that even a giant like Microsoft gave up on maintining one.
Not because of them having radio buttons and form controls. Those we had since Mosaic... Even Lynx supports those.
It's all the other craziness (from 3D, to MIDI, to sound APIs, to 2-3 layout engines, to crazy CSS features).
It is perfectly maintained as the exact UI element it should be.