I suspect some of this is due to the fact that the programmers consenting to do free work already have well-paying jobs, so they have the freedom and time to pursue coding as a hobby for fun as well. Graphic designers and UX designers are already having a hard time getting hired for their specific skills and getting paid well for it, so I imagine it's insulting to be asked to do it for free on top of that.

That said, I don't think it's as simple as that. Coding is a kind of puzzle-solving that's very self-reinforcing and addictive for a certain type of person. Coders can't help plugging away at a problem even if they're not at the computer. Drawing, on the other hand, requires a lot more drudgery to get good, for most people anyway, and likely isn't as addictive.

Open source/Free software communities are comprised of programmers. People love to help their communities. Sometimes a community contains some artists, but this condition is rare. e.g., Inkscape have some good picture when user open it.

Why aten't programmers drawing furry porn?

It's really not deep.