You don't really debug a painting. That's why Open Source is big and Open Art isn't (to my knowledge)
I'd bet that isn't true and that plenty of artists can and do peruse places like deviantart and quickly improve upon someone else's images.
But design for a GUI is more like industrial design. Getting a designer to help improve KDE's look-and-feel would be like squatters in a dilapidated hotel convincing an interior decorator to spruce the place up for fun.
Actually, that sounds like it would be a blast. Perhaps open source people are simply terrible at marketing.
In short outside of the bizarre software/tech world very few subscribe to the idea that intellectual property should be shared and used freely.