It’s mostly about buying votes cheaply. There are significant incentives to consistently maintain a food surplus, but one you start handing out government subsidies you get the usual corruption. As subsides crash prices it’s cheaper to just pay farmers to do nothing than to pay them to create a crop that’s just going to rot.

Basically, you don’t want to pay 1$ to farmers per ton of corn just to lower the price of a ton of corn by 1$. The goal is to hand that dollar to the farmers not the general public.