Because there is nothing to compare it to. If he took another shot at, say, f/11, and you were able to see it at 100% on your screen, you would see it is sharper. Besides, we are talking about a book reproduction (offset?) at a drastically reduced size (as compared to pixel peeping). Under those conditions, almost everything looks good enough. Also, defraction effects can be sharpened away with a proper technique (deconvolution).