Its a matter of false statistics. By doing this sure IF the caravan gets hit only a small amount is at risk, but you have also increased the risk of ATLEAST one caravan being hit. Really is there a reward here? I doubt it.
When you gamble, do you make one huge bet, or do you make many small bets?
Let's say that there's a 10% chance of your caravans getting hit, and you have 100 bushels surplus every week for 10 weeks--total of 1000 bushels.
You could send 10 caravans , and lose one.
Or, you could send 20 caravans, and maybe lose two. It's the same amount of food lost. But, maybe you'll only lose one, in which case you cut your losses in half.
Or you could send one caravan. You probably won't lose it, sure, but if you DO lose it, you'll hate yourself for it forever.
"Don't put all your eggs in one basket."