I read an interview with the maintainer[1] and it sounds like he's put in a lot of work but as he says it's a "labour of love". I wish someone was paying him, even surreptitiously!
[1] https://libregraphicsworld.org/blog/entry/meet-seashore-free...
A designer that can't code will never start a software project so I guess that it's uncommon for them to get involved in one for free.
Then there are developers and designers involved in open source because their companies pay them for that. Gnome's designers are listed at https://wiki.gnome.org/Design#Team_Members
Two of them work at Red Hat, one at Purism, I didn't find any immediate affiliation for the other two.
Is there any company employing them ? Because i find the user interfaces from the 80-90 even 00 much more usable that the today's crap. Remember help buton ? Remember buttons ? Why does windows 10 looks the same and behaves worse than windows 1.0 ?
And it still hasn't been fixed.
I'm not a big believer in conspiracies, but if there's one I'd not dismiss out of hand, it's that Adobe or some other company has been ensuring that GIMP has never been improved or become a viable replacement for some PS users.
There is obviously a large potential market for a lower cost option for light users of Photoshop who don't want a monthly subscription to Creative Cloud.
Maybe they secretly paid off open source devs to obfuscate the code so much that any potential volunteers would have too much trouble finding a way to re-architect the UI without years of unpaid work.
When I see so many great improvements to complex software released to the community on GitHub, along with the potential for some startup to fork GIMP, fix its UI, and charge some sort of support fee like a lot of companies do with OSS, I just find it very strange that GIMP's UI is still in such bad shape, after two decades oconatant complaining by users.
It wouldn't surprise if Microsoft did or does something similar with the OpenOffice code base. So many compatibility and usability problems that just seem to langusish for decades, while you'd think some company could find a way to make money fixing some of the biggest issues keeping light users of Office 365 who don't want to pay subscriptions.