Here's my theory, and it is a simple one if you think about it. The biggest difference between online advertising and offline advertising is in the focus. In hockey, the focus is during the game. Players are swapped in and out, shifted around, and plays are called. After the game, the coach analyzes what was done right and what might be improved.
Online media works in much the same way.
New sites and creative can be rotated in at any time. Performance can be tracked, and playing time can be increased or decreased based on performance. The focus is on everything up to the game. Just like figure skating, the majority of time is spent preparing for the big day. Once the insertion order is signed, a media planner's work is essentially done.