There often isn't enough food in the caravan to feed all the people for the period before the next caravan. Internal food security remains an issue.
p.s. I don't think you've understood my first comment. In my current game, my tribes require 143 food per turn. This is "1.0 turns of food", NOT 1 food. In order to feed them for, say, 15 turns between caravans, you would need to buy 143 x 15 = 2145 food, at 2 gold each = $4,290 gold. And in any case, there isn't that much food to buy (although, if you have an efficient process of slaughter, for example, you can often buy 'live exported' animals and butcher them for quite a lot of food.)