We eat too much when we are happy. We eat too much when we are sad. Food is connected to our emotional state. There is a lot less social pressure to maintain a lean weight in the Midwest and in rural areas. When humans were first around on the planet earth, those that survived did so because they were able to store fat during the summer when food was plentiful and have that fat to burn off during the winter when food was scarce.
Evolution reinforces the body's urge to eat the things it needs to survive.
But then humans learned to control their environment, we developed agriculture and damned up water supplies. But, our body still operates as if we need to store fat for the lean times. Whenever a person loses weight, a portion of the brain called the hypothalamus responds by increasing the appetite and slowing the metabolism so that the body can store more fat. By contrast, when a person gains weight, the hypothalamus decreases appetite and speeds metabolism. The cost of food in the United States is cheap because our nation is blessed with lots of rich farm land, ample water, and a mild climate suitable for growing numerous crops AND our agricultural system has been very effective and federally subsidized.