A candy bar is a superstimulus: it contains more concentrated sugar, salt, and fat than anything that exists in the ancestral environment. A candy bar matches taste buds that evolved in a hunter-gatherer environment, but it matches those taste buds much more strongly than anything that actually existed in the hunter-gatherer environment. The signal that once reliably correlated to healthy food has been hijacked, blotted out with a point in tastespace that wasn't in the training dataset - an impossibly distant outlier on the old ancestral graphs. Tastiness, formerly representing the evolutionarily identified correlates of healthiness, has been reverse-engineered and perfectly matched with an artificial substance. Unfortunately there's no equally powerful market incentive to make the resulting food item as healthy as it is tasty. We can't taste healthfulness, after all.