It doesn't, because in order to min/max your city (and when it gets down to it, this is essentially the point of the game), you need to trade with neighboring cities, otherwise you're limited by what your (arbitrarily small) city can provide.

It's a broken game design in single player because it is based around a multiplayer requirement. In terms of being able to play offline, you would need something to simulate cities from other players.

Not that it isn't easily fixable if EA wanted to, but it's not exactly playable as designed when offline.