Because food is a resource. You can't just put more money in and get back more apples than exist in the world, maximum food production is just that, maximum food production.
That might be true, but one could argue that after an investment, people would more readily sell the stocks they still have at home
It's kind of simplistic to think all the food produced goes into the warehouses of a region anyway. Looking at it from a medieval perspective, I think the food in the Lord's warehouse consists of a percentage of the total production in a region, given to him by the peasants in exchange for the protection the Lord provides
(I don't know the right terminlogy in english, so excuse me the unorthodox way of putting it
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