I think art is probably the wrong analogy, as UI has a lot of science and research behind it, but I see what you mean. I think UI is hard because it's hard to "prove correctness." With software engineering, correctness is enforced at many levels - is the syntax correct, does it compile, do the unit tests pass, do the integration tests pass, does it deploy correctly? All these things can be answered via a yes or no question. UI, on the other hand, is only proved "correct" through repeated use of the program, discussions with users, distillation of that feedback to new UI changes and then more repeated use, discussions, etc. And even then, you're not really sure if you got it right. You just have to use the damn thing and see if it "feels" right. And even then, what "feels" right to one person may not actually be right. Or what feels right today may actually feel wrong after weeks or months of using the software. It's a hard problem.