seems to me that, if this makes any sense, the interesting question is: why can't you have both?

given that selection for "great art" is over a large number of people, why can't you find someone that is gifted in both areas?

perhaps one somehow counteracts the other (for example, you are praised for "mechanical skills" and so never learn "abstract thinking")?

or perhaps the market values the combination of "poor mechanical skills" with "great abstract skills" because it places the latter in strong relief?

or perhaps you do get people who are both (gerhard richter is the best example i can think of), but that's not enough to meet demand? (or meet fashion?)

or perhaps the numbers just don't work out - the population is just large enough for one or the other, but not both?

or, finally, perhaps the whole basis of the post is, in fact, false?