I've thought a lot about this. I think it goes much deeper than people merely not being great at interface work-- I think a lot of FOSS development is a backlash to the sort of development people have to do at work, where they're forced to reckon with designers that have more say about what the interface looks like than they do. Also, in many instances, these developers tell themselves that they are making great interfaces but they're just a little bit ugly, and people need to read the docs and get over it. Most computer users who use dozens of application every week will never read a single complete paragraph of software documentation in their entire lives. Why? Because they don't have to-- and many of these FOSS applications are doing things a lot simpler than what MS Outlook does.

Also, anybody who's been to art school or mentored junior developers knows that we get most defensive about the things we're least confident in. Unfortunately, that comes out when trying to address UI problems in FOSS projects.