Growing corn for ethanol is mostly political. Iowa grows a lot of corn, and as the first state in the political primary process, it gets way more attention than it deserves.
So the corn farmers are sacrosanct. We can make various mumblings about energy independence and surplus food capacity, but we all know that the real reason it remains is that anybody who proposes doing otherwise would get massacred. (Not just individually. Their entire party would take the blame.)
But you're right. It's entirely political. It's not clear why it needs to be. Can farmers really swing that many elections?
Why not pay them to fallow land instead? I remember Catch-22 had a passage describing it, but I have no idea if that's true IRL. It preserves farming skills, labor, and farmland, and gives farmers free money. Political slam-dunk and a boon for food security.
I think that's why we don't just do more of that: it's kind of embarrassing. Farmers don't want to hear just how little they actually matter.
That still doesn't explain why we're so busily kowtowing to farmers. I suspect a fair bit of it is inertia: it's the accepted wisdom that insulting farmers is bad (and telling them that they don't actually need their subsidies is an insult). There may well be a day when some political candidate goes to Iowa and says, "Eff you and your stupid caucus. I'm going to spend my time in New Hampshire, and tell them how I'm going to cancel corn subsidies and use the savings for maple syrup subsidies".
So the corn farmers are sacrosanct. We can make various mumblings about energy independence and surplus food capacity, but we all know that the real reason it remains is that anybody who proposes doing otherwise would get massacred. (Not just individually. Their entire party would take the blame.)