Maybe I was assuming too much or maybe I didn't state that completely. Of course defenders must and do pick up receivers as they enter into their zone, becoming "man" coverage during that time period, if you will, until the receiver leaves their zone. Just pointing out that it is inherently perhaps more difficult to play a zone correctly mentally speaking; there is a lot more that goes into identifying and covering the receivers from an individual and a team cohesiveness perspective. It's much easier from a mental standpoint to just man cover the same guy throughout the play. Most times when a QB beats the zone it is either because the receiver found the window before the closest defender can get to him, or because of screwups/miscommunications on the defenders' part. We have had ample share of both and of course the longer the play goes on, the more likely it is that we will suffer a defensive breakdown somewhere, whether we are in man or zone. If you can't see that then I can't help you.
Taking your Cover 2 example, this is why the middle of the field is typically open against Cover 2, especially if you have for instance a long sideline route in that same play. The two deep safeties must choose who they are going to cover and at some point there is high probability that one of them will miss, or will be covering someone else, when the go route enters their zone, and is left wide open.
A quarterback like Manning -- especially when not pressured -- only needs a very small window of opportunity to hit his receivers. Which is why I repeat; pressure him. It's what every defense has done to beat him. The ones that soft zone him for most of the game get numbers like 400 yards passing hung on their heads.
