To broadly say that "art" is purely subjective and UI/UX is not is just a bad analogy. No matter what form of art you are doing, whether it is the art of UI/UX, painting, composing music -- the professionals of the trade use experience, technique and other tools to create more sophisticated designs that work and are successful because of the use of those tools and the planning/science behind their method. A more appropriate analogy would be to craft or hobby work, not "art."