Because their physiology is adapted to their environment/diet. They are able to properly regulate their energy intake, expenditure, and storage. However, take one of those wolves or their relative, the domesticated dog, and put them in an 'artificial' environment (non-native diet, altered sleep patterns, new stressors, hyper-stimulating surroundings, etc.) and they have metabolic problems.
Humans are the same. Give a human a non-native diet (standard American diet) a hyper-stimulating surroundings, altered sleep patterns, chronic stresses, etc. and they will have disrupted energy metabolism too. Human physiology is not adapted to handle this modern environment/diet.
All of the quibbling about Calories, carbs, fat, exercise, and weight loss is irrelevant when viewed in this context. Our physiology will properly regulate energy stores, appetite, and expenditure when we are in a native environment (or a modern version of it). We don't have to turn into cavemen, but just understand the environment/diet in which our physiology evolved. If we are able to live in this context, then our physiology will be optimized; energy intake, expenditure, and storage will be properly regulated.