I know lots of design folks that contribute to FOSS projects-- they just contribute as developers. The "unicorn" moniker for hybrid designers/developers is bullshit. You might not get a designer that's going to rewrite your embedded system firmware, but I'm a college-trained designer that was a full time web developer for 10 years.
None of the other designers + experienced coders I know contributes design to FOSS projects because the process is just so miserable. You constantly have to justify the very basic value of design contributions only to have it rejected, or completely chopped up by someone else who has no understanding of what you conceptually contributed. It's completely demoralizing.
And as a long-time developer, I get the frustration with design. Sometimes design choices seem completely arbitrary or superfluous to developers... though the root of that is developers often a) assume they know enough about design to critique it, and b) assume that design is purely aesthetic when UI/UX designers often don't even consider aesthetic concerns even if they have related training-- it's all about workflows, telling users what they need to know to solve their problems while keeping the cognitive and visual load low enough to not slow them down, and giving them the appropriate controls to do what they need to do. If your crowd is developers, then the interfaces might even look like what the developers would make-- their mental model essentially equates the GUI to a thin wrapper around a back-end API which actually does the work. To the 95% of other potential users, the interface is the tool. Interfaces are all about communication, and much the same way technical writers are way better at making end-user tutorials than developers are, designers are way better at figuring out how to communicate functionality, intent, and information to non-technical users.
Now that you mention it, I do know a couple designers who do FOSS development but I doubt they would be interested in doing design or research for someone else's project.
As someone who also jumped from development to design I agree with your description of the friction between developers and designers.
I'll also add that compromise is especially difficult when everyone involved is a volunteer, many of whom seem to be attracted to FOSS partly to reclaim some of the autonomy missing from their day jobs. And when projects become popular many maintainers are petrified of making hard choices that might anger existing users.
Given that, it's not surprising how many FOSS projects fall back to tinkering with icons and colors but otherwise recreate workflows from proprietary competitors.
None of the other designers + experienced coders I know contributes design to FOSS projects because the process is just so miserable. You constantly have to justify the very basic value of design contributions only to have it rejected, or completely chopped up by someone else who has no understanding of what you conceptually contributed. It's completely demoralizing.
And as a long-time developer, I get the frustration with design. Sometimes design choices seem completely arbitrary or superfluous to developers... though the root of that is developers often a) assume they know enough about design to critique it, and b) assume that design is purely aesthetic when UI/UX designers often don't even consider aesthetic concerns even if they have related training-- it's all about workflows, telling users what they need to know to solve their problems while keeping the cognitive and visual load low enough to not slow them down, and giving them the appropriate controls to do what they need to do. If your crowd is developers, then the interfaces might even look like what the developers would make-- their mental model essentially equates the GUI to a thin wrapper around a back-end API which actually does the work. To the 95% of other potential users, the interface is the tool. Interfaces are all about communication, and much the same way technical writers are way better at making end-user tutorials than developers are, designers are way better at figuring out how to communicate functionality, intent, and information to non-technical users.