It's difficult to get pros to donate work. Even when they do, the work they do can be ... less than perfect ...

In the project that I developed, and turned over to a new team, we finally (after about ten years) got someone who is a professional UX manager/developer to step up, and he's been doing an awesome job.

In my case, my personal motivations run deep. I do a "better than pro" job on all my open source. I'm a passably decent designer and UX guy, so that helps.