Ease of use by the vast hordes of hundreds of millions of completely non technical endusers and thousands of person-hours by highly paid UI/UX designers have trumped decentralization and autonomy.
point, click, drool.
Blatant UI bugs often don't get fixed in open source. There's a bug in Ubuntu on some machines which causes the cursor to disappear after coming back from a suspend. (You can get the cursor back by typing CTL-ALT-F1 / CTL-ALT-F7.) The bug has been reported repeatedly for five years.[1]
With enough code bloat, all bugs are deep.
[1] https://www.google.com/search?q=ubuntu+cursor+disappears
I also think it's completely fixable, because all the reasons to do open source programming work apply to UI/UX work. Getting your work in front of a lot of people, gaining experience and notoriety, getting to scratch your own itch.
There just was no Linux for design people to jump start that culture. And Mac's always had such nice design that they didn't have as many itches to scratch.
Good UX/UI people don't bother to pay for their tools, like in most professions, and prefer to work in software like Photoshop, Illustrator, Sketch, Maya, Poser, you name it instead of constrain themselves to "worse is better" just to feel good.
So in the end, most don't feel the need to join any FOSS community and rather build their portfolio on online design galleries and magazines.
Doesn't Adobe continue to charge big $$$ for Creative Cloud? I don't understand how these statements mesh..
"Don't bother" as in like most professionals they see a value in paying for their tools and don't mind having to pay them.
There are some notable exceptions of things that have great https/browser based UI: Take a look at Rainloop webmail for example.