Because WinUI and QT are so glamorous and offer a super intuitive development experience. /s
As for WinUI 3, apps are supposed to bundle it, so if you target Windows 11 and run the app on Windows 10, it'll look all rounded and drop-shadowy instead of flat like native Windows 10 apps. Great stuff.
Not even portable Web code, they rather ship a copy of Chrome, and then complain Google has taken over the Web.