Their mother's milk, during infancy. No mammals other than humans drink milk after infancy, and humans have only been doing that for a few thousand years. I'm sure there's links to evidence I can find but I'm not going to waste my time as it's common sense.
Because the average adult doesn't need such a high concentration of fat and calories in a single drink / bowl of cereal / piece of cheese etc. If it was healthy and natural you'd be drinking your mother's milk as an adult, but I'm sure you aren't.
