We developers write something for free because we need it. Even if we build something awful we still use it and maybe open source it. Then maybe we improve the UI but it's not our job so we're not good at it.

A designer that can't code will never start a software project so I guess that it's uncommon for them to get involved in one for free.

Then there are developers and designers involved in open source because their companies pay them for that. Gnome's designers are listed at https://wiki.gnome.org/Design#Team_Members

Two of them work at Red Hat, one at Purism, I didn't find any immediate affiliation for the other two.