They need to cover what is going to happen in a 6 part adventure that will fill many sessions. (Although I doubt the 3 part APs have introductions that are any shorter!)Why oh why do I find Pathfinder AP campaign backgrounds to be long and complicated?
So the background feels the need to explain why the PCs will eventually end up in location X, meet monster Y etc. (And there might be some reverse engineering going on as well, in that if one of the authors comes up with a cool scene then it might get foreshadowed in a revision to the introduction.) It helps the GM to tie all the various parts together.