On the contrary, we overeat precisely because we listen to our bodies.
Evolution tuned us to survive in a world where food is scarce and periods of starvation are frequent. In the rare occasion that you have excess food, you eat it and build up fat reserves. The saved up fat will help you survive the period of starvation that undoubtedly follows.
So why isn't there a decline in appetite as the fat reserves grow? It won't matter while there are no side effects of adding more fat, but there seems to be a widespread belief that fat causes all sorts of health problems. At the least there is decreased mobility (more likely to be eaten by lions), earlier mortality (eg heart attacks) etc.
I wonder if there are any studies correlating obesity levels with reproductive success, and success of the offspring produced. That is what should end up controlling the genes and behaviour behind all this.
Evolution tuned us to survive in a world where food is scarce and periods of starvation are frequent. In the rare occasion that you have excess food, you eat it and build up fat reserves. The saved up fat will help you survive the period of starvation that undoubtedly follows.