HTML was never intended for dynamic interfaces. Whenever you diverge beyond what it does best which is hypertext, things become more and more hacky and awkward. If you wanted to design an os UI in electron, you would use a framework called react, and you would use a language called JavaScript that compiles down into typescript and you'd need a whole build ecosystem and connect everything together, then you might find HTML to awkward so you have to switch to webgl which is too low level or you might switch to svg... In both cases you miss the text rendering from HTML. If you stick with HTML then you'd be using css to move shit around and create windows which is also incredibly awkward.
People use electron because it's easy. The reason why it's easy is because of familiarity not because of actual superiority.
Web UI is one of the most hacked together technology stacks ever. Developers with nothing better to do keep coming up with new abstractions trying to build for a moving target. It's not unique in this regard but it doesn't change the reality. Godot has the benefit of a very specific design implementation for a target that is highly dynamic and likely will never change. Plus it's new so you don't have armies of developers trying to "abstract" shit on it over years and years of horizontal progress.