The problem is that the current developer base of many FLOSS project don't understand that a) decent UI requires an expertise and b) knowing how their poor UIs are implemented doesn't count as expertise in UI development.
For example-- if I write to a FLOSS mailing list that the way they are using realloc leaves some stale pointers that can cause crashes, they very quickly fix the problem. But if a UI expert posts something about a similarly severe and easily fixed UI bug, they'll get at least 5 authoritative-sounding responses from people who know absolutely nothing about UI design.
Such a UI developer would definitely need a full-time salary to deal with that kind of social situation.