There is a (1 gold) cost to sending each additional caravan.
And there's the added benefit of maybe not hitting a wall on the other end--spreading out your caravans means that they have a chance to eat the food or send it further on or at the very least set up more buy offers.
And conversely sending out many over time might mean someone else has time to fill the orders, which you could have filled had you send a larger amount out. These simply AREN'T absolutes we are talking about.
