I agree with you here. As a designer, I have helped out FLOSS projects with UX design and usability. However, the environment is such that if you would suggest UX could be improved, the response often is that devs feel insulted because you are dissing their code (which is not the case, but they don't see that).
On the other hand, taking a look at WordPress, they invested quite some time in user testing and improving UX by professionals, and that makes it one of the most usable web publishing platforms out there.
So, it would be too easy to say all FLOSS is bad at UX, but most developers don't even see it as an actual discipline, and hence do not see a problem.
Nicer devs, that's the key :)
Since you are here, I think you probably also understand that sometimes a complete redesign amounts to a large quantity of boring work, and that an open source developer might have set other priorities for future improvement, or just not see that much return on the time investment relative to other things which remain to be done.
The word for someone who is really nice to you and changes the whole project at your whim is 'employee' or 'contractor.' If I devised and have worked on an OSS project for years and some guy comes out of the blue basically saying that I should reallocate months of free labor in order to make something to someone else's tastes, can you not see why that would be the sort of suggestion I would take with a big grain of salt?