...are superior to the formless, abstract backgrounds in paintings by Boris:
The question is: how do we know?
We can't say that one is better because it is more accurate or realistic. There are no objective laws, similar to perspective or anatomy, for judging soft fields of color. We have no external reference points, like the ones we employ to evaluate facial expressions or poses or outfits, to help us understand which looks more "right."
If anything, in the following comparison, Frazetta's undersea background is less accurate:
Whatever the reason-- intuitive or biological or organic-- we somehow know that the random brush strokes and shapeless colors of one artist are consistently better than the random brush strokes and shapeless colors of the other.
I am offering this comparison for my friends out there who complain that modern abstract painting, such as color field painting, has no viable standards of quality. They grumble, "How are we supposed to distinguish between a good abstract expressionist painting and a bad one, or even a random paint spill?"