OP wrote: "I would have been far more qualitatively useful to society at large." There are far more people that need UI work (in the form of websites, apps, etc) these days than art. UI work scales very well, art not really - you can affect way more people and that is reflected in the salary.

"Qualitatively" UI work is far more useful to society.

There's absolutely no basis for a judgement like that without introducing assumptions about what's valuable and what's not.

Earning money is not easy. The fact that on average a larger group is willing to pay for UI work than for art indicates that UI affects their lives more than art and is more worth to them. Whatever one's reason is.

Let's go with "valuable is something that improves your life" or "improves your wellbeing"

That doesn't really work. Is my life better or worse if I feel worse about myself because I become aware of the way that I'm part of a unethical social system? Given the choice, I wouldn't choose to be a happy person who lives in the Jim Crow south and supports segregation. Part of what many artists do is help people understand themselves differently. You can't assume people are going to be willing to pay for something that's good for them (by their own standards).