I don't think mcdonalds advertises because they have a stockpile of corn and nothing to do with it. Food vendors advertise because more purchases mean more profit. The government subsidizes the corn producers basically whether or not they actually sell anything. The entire purpose is to keep them on standby if we ever somehow become completely cut off from the rest of the world.

I'm pretty sure mcdonalds and frito lays would still be advertising even if their input resources quadrupled in price.

Everything is made with corn syrup because it is cheap, because it is an easy use for corn. If we made it easier to turn into other products like beef feed and ethanol, or less profitable to use corn syrup like with a tax, then there would be less corn syrup in food, but anything that doesn't provide an easy use for the oversupply will not work, because it just doesn't meet the desire of the US government to keep an obscene oversupply of corn in the market.

Government doesn't need oversupply.

It is just historically rural regions in the US farmed corn, and it was enough for the US and little bit of export.

But over time agriculture effectiveness has significantly improved due to: fertilizer technology (CF Industries), agricultural equipment (John Deere), seeds technology (Monsanto Bayer Dow Chemical), food processing.

Thanks to that, the yield has significantly improved and amount of corn produced has increased dramatically, even without increasing land

Think of Moore's Law, but applied to food instead of microtransistors