While that is true, the more interesting question is why some people are driven to consume a large number of calories despite the desire to be leaner.
Also consider that even obese people tend to reach a balance in their calorie consumption and expenditure even without a change in diet or an increase in physical activity.
Let's phrase it more precisely then: Populations of human beings do not grow obese by consuming relatively high proportions of dietary fat. They grow obese by consuming relatively high proportions of starch and sugar.
If you chained a person to a bed and stuffed them with lard every day, you could probably make them obese. But that's not what we are talking about. People who are eating according to their appetite get fat from consuming too much sugar and starch, which screws up the whole endocrine system. They do not get thrown out of wack and end up consuming too much food when they feed on fat.
Also consider that even obese people tend to reach a balance in their calorie consumption and expenditure even without a change in diet or an increase in physical activity.