US government subsidies make processed, unhealthy foods much less expensive than healthy whole foods.
In part 2 of this program, Peter Jennings explores how agricultural subsidies have a direct connection to obesity. The food is cheap to produce and can be sold at high prices because of branding.
Politics, not what is best for the health of the US or the world, is behind the decisions that affect the food supply. That's not really a surprise to anyone who has paid attention.