I don't think this is really the core issue. If you're not getting paid to work on an open source project, chances are you're doing it just because you simply love
programming. You probably aren't an expert designer, and, more pertinently, you're probably more than happy with what you have.
In my experience, building a UI makes you blind to its shortcomings. There are methods and procedures around this, but if you just love programming, chances are you don't know them. I found this out myself when I saw my dad struggle to use even a simple chess game I designed. When you've made something, you naturally know it inside-out; nobody else is going to have nearly that much experience with it.