I like improving UX but when I look at many open source projects from the outside they are run by cranky ubernerds who triumph technical merits above everything else and I can't imagine them being very cooperative with someone whose specialty was "UX."
Sounds like you've never actually tried.
I maintain an open source project (https://conversejs.org) that could benefit greatly from the expertise of a UX pro. However, to date no-one has approached me to help with this (specifically regarding UX). If you or any other UX people here are interested in helping out, hit me up at https://opkode.com/contact.html
I definitely don't have a lack of respect for UX people, having been fortunate enough to work with someone whom I consider to be a brilliant UX designer.
Good software takes strategy, UX, visual design, solution architecture, software development, QA, DevOps, post-launch support, KPI monitoring & analysis, marketing... and probably a few other skill sets to really have it thrive.
The trouble with many open source projects is that they are one-man endeavors, and the person who started it cares more about control than quality outcome. "This is MY pet project..." is a horrible mentality to take when shopping for a partner.
The moment the dev thinks, "Gosh, I'd rather spend my free time watching a baseball game than making those 50 changes the UX guy asked me to make... especially since all I really wanted from him was a color pallet..." that's about when the partnership dissolves. (Misunderstanding about the different areas of expertise intentionally included.)
One person can't think time is worth more than a teammate's. Good software requires compromise and horse trading and a lot of planning.
You typically need money to keep everyone interested.
There are very few open software engineers who like improving UX. Since it's free you have to like it to do it, so I don't know if this will ever change.