Because Apple and Microsoft and Linux all have their own proprietary UI libraries, and their own Look And Feel, and it's just not worth implementing for both/al

Because Apple and Microsoft and Linux all have their own proprietary UI libraries, and their own Look And Feel, and it's just not worth implementing for both/all unless you are super large. And then, if you are super large, then you actually want to create your own Look And Feel. So the web is like a second language. It's a second Look And Feel for users, and users appear to be comfortable learning the native UI LAF and the Web LAF. For developers, it's a second language in that it's JS/TS/CSS that works across all platforms. It's just easier and cheaper to write for the Web LAF than it is to write for all proprietary LAFs and the results are pretty good, sometimes better. Third party stuff like java, avalonia (cross platform WPF), Qt just suck in my experience. Might as well just use electron.