@FeRDNYC makes some good points about the disadvantages of cross-platform UIs. It would make things a lot easier for developers though, and some users use multiple OSs and would like to have the same programs available on both/all and more than vaguely resembling each other. Without a good portable native UI library available, everything’s going to end up being a web app, or perhaps Flutter.
There’s a new cross-platform UI about to be released, .NET MAUI. Unfortunately, Microsoft aren’t actively supporting Linux with this, leaving it up to the “community”. There’s likely to be resistance, but I think people should get over it. MS is rather different now from their Steve Ballmer days. There is a Linux fork of MAUI on github, but I don’t know if it works yet. It says it uses GtkSharp. So that’s a dead end unless someone replaces it with a GTK4 version, preferably automatically generated from GI for future-proofing, and opening up things like gstreamer.