Actual good UI/UX design isn't trivial and it tends to require a tight feedback loop between testers, designers, implementers, and users.
A lot of FOSS simply doesn't have the resources to do that.
Projects like GNOME, Elementary, Blender, Krita, KDE Plasma, Penpot, and MuseScore seem to attract contributions from designers.
I suspect it's because designers are like any other open source contributor: they want to work on projects that they use themselves and where their contributions will be appreciated.
How many of them are paid? I know MuseScore, Penpot, and Blender have paid for almost all of their design work (because they have paid staff)
Not just a relevancy problem, it's much easier to get free development work in OSS than design work. It's a decades-long problem.
I'm still skeptical that it's primarily about pay. I know many designers who do pro-bono design work, just not for FOSS. They typically work on nonprofit websites, community newsletters, contribute game mods/assets, work on civic tech, even band posters.