I personally think this just has to do with less openness from projects to talk to designers and/or UX designers. Most OSS projects are maintained by a small group that wants to do everything themselves and consider "modern" things as unneeded cruft.

This makes the step for a (UX) designer to propose to help a lot more difficult as rejection is high because besides them giving some input and maybe a few designs they cannot just do a PR where they send over some HTML/CSS that makes it better.

Also a programmer as a hobby can kickstart a project, find a buddy to help maybe on the UI a bit here and there and they call it a day.