I understand what your saying, and you do make some good points, but I don't think humans should get obese due to lack of exercise. Some exercise is important, but only really important as a muscle builder. Walking everyday is all that should be needed.
Weight with a normal diet self regulating. You eat until you get full, and this is primarily run by the hormone
Leptin. Leptin is secreted by your fat cells, and the more circulating in your body, the more your appetite is inhibited. Unfortunately, carbohydrates block the hormones message, by causing
Leptin Resistance.
As you can see, this sets off a vicous cycle, where the only result can be overeating.
It actually makes me mad that animal fats have been demonised by the media, when in reality, it's the carbohydrates that are causing all the problems. Animal fat is good for you.