You are missing the picture that they provide full development experience stack for desktop development, similar to Framework/Kits/Components on major mainstream desktop and mobile OSes.
Like always many of these experiences fail short by focusing on Look and leaving out the other 90% of the development experience.
GNOME and Qt both offer full-fledged UI frameworks. It sounds like you haven't given either a try before, but they're both quite capable. Programming for them doesn't feel any different than writing a Cocoa UI or an Android app or a website. It's just... another target.
Like always many of these experiences fail short by focusing on Look and leaving out the other 90% of the development experience.