I’ve seen many open source projects have bad design. Even large enough organizations, in Linux space for example, have dated design. (Even ElementaryOS is far away from Mac in terms of UI). Also sometimes it seems to me that modern design makes the project feel “not open source”.
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.<p>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.<p>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.