You also can't give away your overproduction to less well off countries, as you cripple their ag market, preventing them from bootstrapping their own economy.
It's a moot point with transportation electrifying as quickly as it is. I don't have enough knowledge to say what crops we should be planting above and beyond what meets human consumption needs, but I can say without a doubt we should not be subsidizing crop based biofuels (only algae based fuels, only to be used for air and marine transport).