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.
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.