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.
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."