Because breast milk and baby formula are complex as all hell. Babies have complex nutritional needs–for example, high amounts of choline supplementation are required in baby formula because otherwise the baby becomes retarded.
A five-year-old child isn’t a baby. A child is growing rapidly for sure; a baby is trying to develop into some semblance of a human bioform. Babies come out because they simply don’t fit anymore. They come out with brains so undeveloped that they can’t support consciousness (they can scream, smile, and otherwise react; but they aren’t actually aware of the world–and humans recognize anthropomorphic behavior as awareness specifically so they don’t recognize a newborn infant as a non-human lump and abandon it). It takes three months for the brain to get into a useful state.
Failing to provide the necessary diet will cause bad development in the worst way. Well, okay, alcohol is the worst–so much so that mothers using heroin in pregnancy typically have much-healthier babies than other drug-using mothers because heroin and alcohol is a fatal mixture (everyone loves alcohol and stimulants, so other drug-abusing mothers frequently consume alcohol during pregnancy). Early diet can still be ruinous, which is why the WHO recommends breast feeding through 2-3 years of age.