Adding on others (lack of recognition/respect, very difficult to submit changes if you're not a dev, etc).

One reason designers usually don't contribute to open source, is because they're paid to improve UX/UI on their commercial competitors. If GIMP/Inkscape had the same UX/UI as the Creative Suite, there wouldn't be market for the latter (this is what designers are paid for, unlike developers).

This allows more startup opportunities or companies like Canonical (parent company of Ubuntu) to provide a free and open source product (Canonical actually has dedicated designers).