There are two main causes. As societies grew in technology, the need for physically demanding labor has declined. And, as a result, more people chose stationary and less active jobs like desk jobs. These jobs don’t require much energy so people don't burn as much calories as they used to, but still eat as much. Leading to excessive calories stored as fat in the body. Resulting in more overweight people. The second complementary factor is fast food. People used to spend great amount of time making food on a daily basis. However, as technology grows, modern production lines can process food at a remarkably fast rate. Thus, fast food can usually be served in less than an hour compared to traditional food. And people spend the freed time eating more fast food that is high in fat and calory. As a result, the overweight population increases.