a. we developers don't make it easy for them to do so.
b. designers don't care about our projects.
For example, look at wordpress ... you can go in and create a theme ... today, if you'd like to, as a result there are tons of free themes out there, but how would a developer go about creating a Netbeans theme?
Also, most open source projects are created because a developer wanted to build something cool for themselves ... and it isn't often that designers find those sorts of things cool as well so of course they're not going to be designing cool ui for those things
The first is solvable ... devs need to make a concerted effort to reach out to designers, outline a design process (send us a psd of your design, we'll go over the details in basecamp/unfuddle/whatever and you can provide direction as we go) ... I can't see much being done about the second.