Mainly because of culture differences.

Good UX/UI people don't bother to pay for their tools, like in most professions, and prefer to work in software like Photoshop, Illustrator, Sketch, Maya, Poser, you name it instead of constrain themselves to "worse is better" just to feel good.

So in the end, most don't feel the need to join any FOSS community and rather build their portfolio on online design galleries and magazines.