The big problem is that nobody wants yet another platform-specific UI framework. Even Microsoft is using electron (or something like it) to build Microsoft Team

The big problem is that nobody wants yet another platform-specific UI framework. Even Microsoft is using electron (or something like it) to build Microsoft Teams. If you build a cross-platform UI framework, it'll probably end up looking a bit ugly and custom on every platform (eg Java Swing). Making a cross-platform UI toolkit that looks native everywhere is an insane amount of work, because different platforms have very different UI toolkits, with different built-in components and different platform conventions. This problem becomes 10x harder if you want to make it work on mobile as well. Some people try anyway - and bless their cotton socks. But electron (and arguably Qt) are the closest we've got. And even then, most electron apps seem to be full of custom components.