Look it's obvious that you have some kind of grudge against designers. For your own good at least see the enemy you respect...
Just because I know that as a general rule of thumb, one person can not be the best of the best in all three does not mean I have a grudge against them. I have some generalist who are excellent in all three, but they are rare and that is a market reality.
It just means I know that there are a lot of hacks that pass themselves off as masters of all three. When the reality is they usually have mediocre skills in HCI if any at all. I have never seen an industry where people pass themselves off as a professional of a field of scientific study, when they have never cracked a book on HCI and have no idea what human factors is. In any other study this would be considered fraud. Just because I know that 80% of designers have no exposure to HCI methods, does not mean that I dislike or have a grudge against them. It just means that they need to either be trained in HCI or not allowed to own that task in my orginization. They are distinctly different disciplines and require completely different education and toolsets. If that depresses you then so be it, but don't accuse me of bias because you don't like or don't know the facts. It is incredulous and a logical fallacy.
It depends, color pallet selection would be an example, we have a specific pallet that we use that works for color blind individuals, that pallet was created by our UX professionals, it is provided to the artist as a requirement of the project. What colors that are selected out of that pallet is a function of the artist job.
A/B testing, what color schemes work well is the function of the usability analyst. As that is not a detail that can be front ran.
These are three distinct roles and sure you can hire one guy and he may be decent in 2 but 10 to 1 he is not going to be top of the industry in all 3. 1 if you are lucky, so you have to make the decision do you want one guy doing 2 sun par secondary responsibilities or 3 guys doing their primary role better than any one guy could do all three. That is not to say that you don't have generalist, we have a good deal of them but when it comes to art, if you want the best generalist is generally not a skill set that you want to look for.