Have you seen the state of Javascript 'ecosystem' and job market?? Both statements are false.

The real reason is that C++ toolkits don't have platform and device independence. HTML and CSS are really good and powerful design tools, especially compared to composing raw pixels like you'd (essentially) be forced to do with Qt and GTK.

Let me correct: GTK+ is not C++, it's C. Qt is very cross-platform: it supports about 10 platforms, including Linux, Windows, and macOS. You are not doing anything close to compositing raw pixels in Qt or GTK+. They have layout managers that are specifically made for typical application UIs, and they have lots of ready-made widgets. Qt also has QML, which is a declarative UI descriptions language kind of like HTML/CSS but much more productive because (gasp) it's actually designed from the ground up for applications, not websites.

Which one would you recommend?

Cross-platform story is complicated, though. I have good experience creating a custom GUI framework for embedded Linux: drm/kms, gles, NanoVG, [DllImport], everything on top is custom C# code in .NET core 2.2. But that approach is not for everyone. I only needed 10-15 screens with simple GUI on them, it’s a 5” touchscreen with no other user-facing devices. Creating more advanced UX this way would consume too much time.

There’re Avalonia and Xamarin. Didn’t used them at that time because I needed full-screen GUI for a single-application Linux, without any desktop environments or user mode OS components. For desktops and especially mobile apps they might work OK.

It’s unpopular opinion here, but I think Electron is not that bad for GUI, esp. when writing TypeScript instead of JS. I think it’s similar story to Unity3D or PHP: low entry barrier attracted inexperienced programmers, then the whole ecosystem is judged based on the output of these inexperienced people.