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."

There is actually a much better but less well-known open source library in .NET: Avalonia. Look it up their gallery of apps. Avalonia is the cross platform version of Windows Presentation Foundation (WPF) libs. It is quite good for desktop apps and many commercial pieces of software uses it.

MAUI apparently has Windows, Mac and Mobile support but no distro Linux support (unless Wine counts). You could use the web stack to be truly cross platform.

Java IDEs have historically been made by maintainers of the language.

Netbeans was a product acquired by Sun, Sun Forte was its "Professional" variant in Solaris, and Oracle still takes care of it in the context of Solaris and Oracle Linux.

Eclipse was a rewrite from Visual Age products, originall written in Smalltalk, by IBM, and IBM keeps being a Java vendor with their own implementations.

I do get the sentiment to some degree. Part of it is that Microsoft does have a conflict of interest as an OS vender. They do need to show that they aren't/won't be abusing that. That does put them in a position where they're asked to go above and beyond as a form of litmus test.