Nor fat, nor carbs are an essential nutritional component. The body can not synthesize certain amino acids, but it can convert protein to both fat and carbohydrates by means of gluconeogenesis. If anything, we should have cravings for sources of protein, not sugars. One could argue that sugars provide the burst of energy for the metabolic pathway that could be make it or break it in cases where that burst gives an advantage. This, and not the scarceness argument, which seems just thin to me, although mainstream opinion.