As infants we have a certain enzyme in our stomaches that break down milk protein (as in breast milk from our mothers). As we age that enzyme goes away, leaving our bodies ill equipped to digest it. Mammals do not generally drink any milk past the age of weaning so really humans are rather unique that we say “fuck nature, give a glass of milk from a cow and a stomach ache.”
Not sure on the stats without looking it up but I think it’s like 80% of adults world wide are lactose intolerant. Humans are very strange creatures.