A big part of the issue is just that UX and dev are two separate specialties, and while some people can do both, there's no clear place in most open source project for someone whose primary contribution is to provide designs for someone else to implement.

The reason is partly that open source projects are a hell of lot of work for their owners, even without UX experts handing them detailed specifications. And it's partly that UX details often individually seem trivial, even though their accumulated weight can make the difference between a good project and a great one.