Adding to this; there is also no evidence that the US's politicians care about financial incentives as applied to the US government.

If it affects them personally then they'd jump like a cat on a stove; but if it only affects the budget they aren't going to respond to any incentive.

If the US was being influenced by financial incentives it wouldn't be doing many of the things it does. Like tax cuts or spending more money.

The monetary policy of the USA is insane these days. Tax cut and spend is here for the moment. Taxes will go up but so will spending...

Food subsidies poll extremely well and the US votes for them. They are one of the least controversial public policies and not because of "powerful donors." Almost all countries subsidize food production and there are strong reasons why this is so. Primarily, countries want a robust domestic food surplus for social stability. High food prices or lack of food leading to famine are one of the highest causes of social unrest.

Ok, even if you are right, they still aren’t concerned with the fiscal impact of the policy.

Yes, because cheap food is not something that would otherwise interest lots of people...

Increasing corn production also does not play to a crucial constituency in the Presidential elections...

This obsession with “rich donors” ignores that voters will also get mad at politicians for the consequences of eliminating those subsidies.

The smaller your donor base, the fewer people you have to make happy. My personal belief is that this is why income inequality is inversely correlated with democratic representation.

Then fix that?

Powerful donors should not exist.