EF Core in that regard is very similar - you can easily use Dapper instead because it builds on the same primitives from System.Data. Neither MAUI nor other UI frameworks have access to internals of CoreLib or adjacent packages living in dotnet/runtime.
In fact, having multiple libraries solving a particular task is a sign of healthy ecosystem, too much centralization more often than not tends to be harmful.
The reason why Avalonia and Uno have support for Linux but not MAUI is purely technical one - Avalonia and Uno draw their controls by themselves, like with Skia(sharp). MAUI tries to be a successor to Xamarin and uses native controls drawn by the host instead. Naturally, this has always been a problem on Linux and supporting it is non trivial (what do you choose? Attempt to make it work with Qt, GTK, something else entirely? what about X.Org vs Wayland?). Realistically, supporting mobile platforms first alongside Windows and macOS is much more important and is a far better investment of effort (how many users are there with desktops/laptops on Linux vs macOS/Windows) and there are only so many people working on this. It is perfectly fine for GUI frameworks to take different approaches and pursue different goals, where Linux systems are much better served by Avalonia.
Avalonia exists, and from the looks of it is better than all the crap MS has put out over the years (even on Windows, don't get me started on WPF shutters) so not using it because its not "official" is silly.