I think just as large a part is that it's difficult for multiple designers to contribute to a project in a meaningful way without stepping on each others' toes. Do designers often collaborate in groups of more than two? (Layer Tennis is the only thing that comes to mind)

If I were evaluating a project I would choose a project with a default Github page over a project done in MSPaint just because there's a good chance that the latter was written by a developer who likes the MSPaint logo.