Oh I think open-source developers care about design these days, but they're focused on getting the core functionality to work and there's only that many hours in a day. I think designers are not joining open-source projects because it requires learning some other skills that might not be interesting to them, like some basic Git, build systems and a handful of unix commands. There are several sites that have designers making mock-ups and uploading showcases of their work. If there would be a way to link those two communities together, some interesting cross-pollination might happen naturally where developers can get some fresh ideas and designers don't have to produce visual designs floating in a void.