Because nobody is investing in native UI development. Electron is proof that if there were a simple to use GUI stack that companies would adopt it.
To me the worst case is trying to develop some small utility like a tool to search in files using regex. Because if you are developing something large, the amount of time you spend dealing with packaging, distribution, etc., is small and you don't care about file sizes. But if I want to, say, develop the app for Windows. That is easy. You get a tiny binary to just opens a form and runs with a double click. No install…
So don't be self-contained. I mean, you depend on an X server or Wayland, right? So why not depend on GTK or Qt being available?
(Of course, it _is_ tricky to be able to depend on any of several versions of these, but still.)