Fat is just calories. Fat doesn't care about vitamins, or minerals. It doesn't care about anything but calories in vs calories out. Your body can absolutely handle calorie dense, low vitamin/mineral foods and not get fat, but there's a whole bunch of other factors that play into it.

Lower income obesity can be linked to the quality of the food, but OP asking why the US seems more interested in processed packaged food vs "simple staples" that other countries cook. The answer there is availability and cultural perceptions (both of what the people should be eating, and of what the observer thinks the people in that culture are eating)