For people in many parts of the world, there is minimal planning and discipline involved in their diet. They might not “eat right” in terms of a diet rich in the best vitamins and nutrients. But they are still skinny. They eat what is available.
We are the fattest country and it isn’t because of country-specific lack of planning and discipline.
We have made life very easy for people. Just like I am doing right now, people sit on their phones/sit on their butts all day. They eat whatever is easiest and it is often high in calories and as mentioned, carbohydrate-rich calories.
People have plenty of time to do differently. It is a routine of laziness both in eating and lifestyle.