When baby rats grow up, their bodies no longer require milk, and they are weaned. Mother giraffes provide milk for baby giraffes. When baby giraffes grow up, their bodies no longer require milk, and they are weaned. Mother pigs provide milk for baby pigs. When baby pigs grow up, their bodies no longer require milk, and they are weaned.
Humans are the only species which drink the milk of other species.
The answer, of course, is that we didn't. Like all other mammals, we require milk of our own species during infancy. Calves double their birth weight in only 47 days. Cow's milk has enough fat to turn a 45 pound calf into a 400 pound cow. Our bodies aren't supposed to do that.