Here a wild guess on things must be working at EA:
I'm guessing that Sony charge money to host the game that can be bought in the Playstation Store and the price must be a combination of clients quantity and gigs downloaded. So, to save money, EA put a smaller installation on the PS Store and put the bigger download into the game which comes from EA servers.
Naturally, EA underestimated what would be needed for connections and servers so us, the customers, get to play a game 5 days later than the day it was downloaded from the PS store, because EA wanted to get the maximum cash possible from a game.
Note that this is a theory only on why the biggest part of the game must be downloaded in game but since everything is money related; I find that this is a very good one🙂