It's not so much that it's not interesting, or that involves working with non-technical people, or that it requires pixel perfect alignment, or that the screen isn't a deep enough level. It's that it is all about working with other people as human beings. It's about empathizing with people who are different than you are, learning to see through their eyes, giving them what they need, and not getting any payoff in terms of interesting technical problems to solve. It's probably the exact opposite of what draws most people, myself certainly among them, to programming in the first place.
That said, I also agree that pretty much anyone can learn to be a good UI designer, just as anyone can learn to draw well. As in any field, talent helps and the very best people will always be the ones with talent, but it is much more important to want it enough to put in the necessary work. If you are a programmer who only cares about technical mastery then you will never be a good UI designer, and, as you said, there are a lot of programmers out there who are mostly interested in technical mastery.
That said, I also agree that pretty much anyone can learn to be a good UI designer, just as anyone can learn to draw well. As in any field, talent helps and the very best people will always be the ones with talent, but it is much more important to want it enough to put in the necessary work. If you are a programmer who only cares about technical mastery then you will never be a good UI designer, and, as you said, there are a lot of programmers out there who are mostly interested in technical mastery.