Because focus shift is by far the most important lens behavior in determining sharpness as a practical matter for real images, with field curvature running a close second in terms of across-the-frame sharpness.
Focus shift and what to do about it are investigated in Making Sharp Images.
Now I’m not talking about the contrast of a 18-200mm f/crap - f/crappier zoom against a prime lens— I’m talking about a good quality prime lens of any brand— Nikon, Canon, Zeiss, Leica. They’re all very good, but many factors are at play in total quality, and focus shift is at the top of my list for practical consideration in making sharp images in the field.
This is especially true with higher resolution cameras like the Nikon D800E, but it holds true at 22 megapixels and 12 megapixels, far more than most photographers realize (“focus is off” = no idea what’s really going on).
And so focus shift deserves a great deal of coverage with every brand. Expect to see my Guide to Zeiss ZF / ZE Lenses grow in this regard (using the Nikon D800E of course), as well as coverage in DAP for Nikon and Canon and Olympus 4/3, and in my Guide to Leica as well.
Is this the kind of sharpness on the eyes you want? At f/4 where they should be bitingly sharp?