To be fair, when you strip away our higher cognitive functions, we survive like other animals survive. For most dogs, you can't just give them food and let them free-eat - their instincts tell them to keep eating and they will get very obese. With our higher cognitive function we can override storing energy as much as possible because we know our next meal will come fairly soon, but at our base we are animals and we have to actively override it.
Plus, sugar and fat are just so tasty. I wonder if tastiness is a matter of genetics or socialization.