I've seen this comment (most recently in this question), or similar, in several answers here on photo.SE: "Pixel peeped images aren't likely to look sharp".
Why is that? It would seem to me that if you go into Live View, full magnification, and then manually focus on the area of interest, then that is as sharp as it is going to get. Of course, one must assume a stationary subject here, and you have to choose you area of interest carefully, but I keep seeing comments like that one which really imply that I'm missing something here.
So why doesn't pixel peeping with magnified Live View give the sharpest possible image?