There were cross-platform C/C++ libraries in the late 80's to early 90's like Visix Galaxy. However, it was not free - far from it.

We developed on Windows NT and deployed onto HP/UX and Solaris.

Uh Qt is still around. Licensing might be as bad as this Visix Galaxy though these days.

certainly not valid for the most used electron projects

Also most native UI frameworks don't provide the primitives for modern UI. It's mostly just standard UI elements, sometimes with the possibility to theme them.

But most frameworks don't come even close to CSS and all the possibilities to create custom controls. It often requires to build everything from scratch in code, which is really hard for more complex layouts.

One could reply that it's a feature, since native UIs are more uniform and with less surprises you can concentrate on whatever the fuck you're doing instead of wondering what this app's designer was smoking.

Depends on the UI framework. Most frameworks don't have enough features to make the UI actually good. It's not even about looks, much more about layout. CSS is really good with conditional layout: hide something and everything reflows around it, responsive layout (not just mobile, but also when making app windows smaller/larger). Typography is another issue with a lot of native UI frameworks. Even adaptive DPI support is not yet easy with most native frameworks.

Also things like accessibility. It's quite easy to make web ui accessible, much harder to do it with custom native control.

Maybe they should take the hint and not make a modern UI, given where UIs like that have taken us.