> The more food we can make per unit resource, the more food we have, given that resources are finite. Giving the poor more money does nothing to change that.
In the real world operating under market rules, there's not one rule that determines how much food we have. It's a function of how much you invest in the operation -- land, tractors, farm labor, irrigation, livestock feed, food processing, et cetera. People know how much food to produce and how to produce it because of PRICES. Higher food prices are encourage people to reduce food waste (by making it more expensive, making concepts like "leftovers" or "canning" more attractive) or divert resources towards producing food (e.g. by starting new farms, buying better farming equipment, or starting a garden in their backyard). It also discourages inefficient forms of feeding people by raising the price (e.g. making meat more expensive because it takes a boatload of resources to feed a cow). Lower food prices reduce the profit of farming, discouraging the over-exploitation of the land or over-investment in expensive capital like tractors.
In a subsidized environment, the real cost of things is detached from the sticker price at the supermarket. Food is still expensive, but it's not the consumer who's paying for it -- it's the taxpayer. So farms buy more heavy equipment than would naturally be efficient and burn more gasoline than is efficient, food processors waste more food than is efficient, people toss food in the trash more than is efficient, people grow rice in the California desert instead of importing it from somewhere that actually has rain, and midwestern farms destroy more prairie lands and Nature than is efficient, because why not? Taxpayers are paying for it, all crazy-indirectly, and they'd have to go through Congress and go up against big agribusiness lobbyists to get any of their money back. (Then the taxpayers find that they can't pay for other nice things they might want, because of the money going to taxes, and the economy suffers.)
By giving people who need money, money, instead of giving producers a subsidy, you avoid all that. Everyone eats, farms produce the right amount of food with a sane amount of tractors, big factory farms aren't given money that funky forest-culture farms aren't and so each style of farm prevails on its own merits, people with backyards start gardens and make preserves, et cetera.
TLDR: screw with the market's price mechanisms and you screw everything up
Postscript: This is all the fault of the FDR administration when you get down to it.