The Chinese don't drink much milk, either. If you live in North America or Europe, that probably comes as a surprise. Point out North America and Europe on the map. In these parts of the world, most people, including many adults, drink milk and everyone knows how good it is for you. In the rest of the world, however, most people older than about three years have trouble digesting milk.
Milk is a complete food for baby mammals, the group humans belong to.
But after a calf, puppy, etc. grows too old to nurse from its mother, there's no place for it to get milk to drink. So its body stops producing the enzymes needed to break down the special sugars in milk. Originally, that's what happened to humans too. But when some people started taming and keeping such animals as cows, goats, and reindeer, they had a new source of milk. Most still couldn't drink milk straight without getting a stomach ache because they couldn't digest the milk sugar.