Today, most people's work requires little or no physical activity. Cars and public transportation are so available that walking and bicycling are no longer common modes of transportation. In addition, our style of preparing and eating food has changed completely. We used to prepare food from scratch in the home, and eat most meals at home. Today, most people eat a substantial portion of their food outside of the home.
Food is no longer scarce, and is in fact universally available and widely advertised.
Huge industries compete with each other to make their food more enticing, more available, and cheaper than that of their competitors. Advertisers are not under any pressure to sell healthy food, so they advertise what will tempt people to eat as much as possible. While some fat people are unhealthy, there are some people who can be both fat and healthy. Fatness by itself is not a disease, but rather a risk factor for certain chronic diseases, such as diabetes. That means that in a group of fat people, you will find more diabetics than in a group of thin people.