They tried, by forking Xamarin Forms into MAUI, and even then they ignored Linux. It's really rough though, to the point many projects use it as just a glorified webview for Blazor. I expect it to eventually go into a silent maintenance mode along with WinUI 3.

Avalonia is the go-to library for cross-platform UI in .NET right now. But Microsoft's own apps have been switching to web stacks, in a clear case of "Do as I say, not as I do."