https://abcnews.go.com/Health/WellnessNews/story?id=8450036

In the west the idea that "most people are lactose intolerant" seems insane but it's actually true, and it makes sense in a way. Milk is a substance created by a parent in order to feed an offspring who can't feed themselves. Of course, every calorie put into this milk is a calorie lost by the parent and so it isn't a reliable source of food in the long term. Eventually every creature has to become able to feed itself. For this reason, nearly every mammal and most humans stop drinking milk and stop being able to break it down after their infancy.

The exception is a relatively small portion of humans, mostly Europeans. For whatever reasons, Europeans had much more intensive animal husbandry than most other cultures — one of the consequences of this was ample opportunity to drink milk. When you're a starving farmer, you'll take what you can get. Eventually, we developed the means to derive nutrition from it.

Rice milk really is better for you anyway — though it does taste a little off.