Or perhaps there are too many designers? Or overconfident programmers who think they can do the design work?
Deviantart was a huge platform where people would share art and designs/skins. To some extent that still exists for iOS views.
I think one thing that's different about the deviantart model was that back then apps used to have skinning interfaces. And then designers could focus on just skinning. You could argue that gnome themes are like that. I would disagree, but also skinning isn't quite the same as doing ux.
UI/UX on Mac is a lot easier than UI/UX on gtk applications or QT applications.
On top of that, I don't think most designers understand OSS. It's basically a different universe and these two world don't often interact.
It's about the users and their ability to understand how to work with your software to address their needs and expectations... not about your code, the designer or the manager. Those people are the product, NOT the user.
If I had a dollar for every programmer and manager that thought they knew better and stubbornly cooked up a crappy MVP nobody wanted, I would be able to fund your OSS.
Right now I have ten crappy MVPs to donate and can put down $20 to sponsor others. I could probably have another 5 MVPs out within the next week if work is slow.