Cross-platform widget engines (Tk, GTK, Qt, wxWidgets, etc…) long predate Electron and have always been robust, stable, well documented, and easy to use. It's not creating a single application for multiple native desktop environments that was difficult, it's writing a single application as both a web interface and a native desktop environment on any platform that's difficult.
Because CSS is constantly changing, inconsistently implemented, and difficult to use, it can't easily be targeted by widget engines (although Qt has tried, and maybe others) and web browsers already exist, so webview environments like Electron are the quickest way to get a cross-platform installable application that can also be hosted on a web server. I wouldn't really call it native though.
I've used GTK in the past and I wouldn't consider it robust nor easy to use. But most importantly, it doesn't look good, especially on macOS. No matter how hard you try, you cannot make it feel native.
But the point is: LLMs should make it easy to generate a native UI for each of the main platform's UI frameworks. GTK should no longer be needed.
GTK is probably the worst of the bunch, and Tk is the most underrated. Everything looks bad on macOS, because Cocoa's API isn't as stable or well documented as Microsoft's DirectWrite and GDI variations nor obviously X or Wayland, which are open source.
Either way, getting something to appear the way you want it to in GTK is worlds easier than doing so in CSS, and it runs much faster, with a fraction of the resources.
Because CSS is constantly changing, inconsistently implemented, and difficult to use, it can't easily be targeted by widget engines (although Qt has tried, and maybe others) and web browsers already exist, so webview environments like Electron are the quickest way to get a cross-platform installable application that can also be hosted on a web server. I wouldn't really call it native though.