The answer to that is well known: Windows division builds WinUI/Win32 as their native C++/COM API, Office division went to React on their path to the web and the dev division fills gaps (WPF) and provides tools for external and internal devs (Maui for cross platform uis).
It is history not the lack of will. At one point the windows division was in shambles (remember vista) and WPF pops up. At another point, the windows and dev division have no answers to the office group (because you know who uses non win tech) so they went react. And then external devs screamed: where is the .net cross platform story so Microsoft acquired xamarin and later form Maui out of it.
It is history not lack of trust. But the outcome is the same: lackluster support for all UI toolkits.