Lots of reasons. Generally its not just corn but soybeans too. Essentially the USA has optimized its supply chain for producing massive amounts of corn and soybeans and bringing them to market. The subsidy is to ensure that USA capacity to produce enough food for its populace irrespective of geopolitics or world economies. We never want to be in a position where we aren’t ramped up to feed our population entirely. If you overproduce, you can always get rid of the excess. If you underproduce, at least a portion of your population goes hungry. That cannot be allowed to happen. The ancillary benefits are things like USAID that use the overproduction as expressions of Soft Power around the world in the form of nutritional aid.