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.
I believe it's more nuanced than that. Artists, like programmers, aren't uniformly trained or skilled. An enterprise CRUD developer asks different questions and proposes different answers compared to an embedded systems dev or a compiler engineer.
Visual art is millennia older and has found many more niches, so, besides there being a very clear history and sensibility for what is actually fundamental vs industry smoke and mirrors, for every artist you encounter, the likelihood that their goals and interests happen to coincide with "improve the experience of this software" is proportionately lower than in development roles. Calling it drudgery isn't accurate because artists do get the bug for solving repetitive drawing problems and sinking hours into rendering out little details, but the basic motive for it is also likely to be "draw my OCs kissing", with no context of collaboration with anyone else or building a particular career path. The intersection between personal motives and commerce filters a lot of people out of the art pool, and the particular motives of software filters them a second time. The artists with leftover free time may use it for personal indulgences.
Conversely, it's implicit that if you're employed as a developer, that there is someone else that you are talking to who depends on your code and its precise operation, and the job itself is collaborative, with many hands potentially touching the same code and every aspect of it discussed to death. You want to solve a certain issue that hasn't yet been tackled, so you write the first attempt. Then someone else comes along and tries to improve on it. And because of that, the shape of the work and how you approach it remains similar across many kinds of roles, even as the technical details shift. As a result, you end up with a healthy amount of free-time software that is made to a professional standard simply because someone wanted a thing solved so they picked up a hammer.
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.
I dispute that claim but it doesn't answer the question. When you have multiple people involved in the community of an open source project, what makes them decide where to contribute, and what makes them decide if they'll use marketable skills for free or not? I think it's an interesting thing to look into.
