The farm subsidies started after the great depression when food shortage was a serious possibility and national security was forefront. While the rhetoric has shifted, the real purpose has not. Yes, the US has the geography to produce vast quantities of food, but that's all useless if no one is farming it. Even with subsidies agriculture is neither lucrative nor appealing for the vast majority of individuals. If the few remaining farmers go off and get better jobs, a huge amount of implicit knowledge will be lost and a lot of infrastructure would be left derelict. This might not be a problem in the short term, America is wealthy enough to import food, but should its suppliers ever try to leverage that position then America would not be able to instantly resume producing the food it needs. Every major power heavily subsidizes domestic agriculture for the same reason.