It isn't hard for the programmers who write this stuff to eat their own dogfood (use their own software just once) and notice how fucking obtuse, buggy and clunky the UI is. Gnome's System Monitor, which so many Linux desktop distros use as their process monitor, is god-awful, even with so many eyes on it every day. It doesn't take a UX designer to fix it, it just takes a programmer who is familiar enough with the source code.
Another example: Gnome Maps has bugs all over the UI (not bugs at lower layers, because it doesn't crash) but you can left click / right click / menu selection your way into trouble very quickly and easily.
People who write open source software are doing so out of their free time, and I bet they get to a certain point where the functionality is all there, and they get bored with testing and bugfixing the useability aspects.
Writing a good UI is more about really caring than design problems. It takes a lot of time that nobody is paying for and it's not fun. That is why the UX with open source is mediocre. Any programmer can look at a commercial product's UI and try to get parity with what they're writing, but they don't, and that's perfectly understandable.
So in conclusion, I'd say in my professional opinion, that we just need a company with deep pockets to sponsor extant open source programmers to put the finishing touches on their work. It's something they're far better suited to get done than a UX designer.