“Approximately 65 percent of the human population has a reduced ability to digest lactose after infancy. Lactase nonpersistence is most prevalent in people of East Asian descent, with 70 to 100 percent of people affected in these communities. Lactase nonpersistence is also very common in people of West African, Arab, Jewish, Greek, and Italian descent” [1].
Digesting cow’s milk relies on a new gene (<10,000 years old) that evolved in Northern Europe, East Africa and the Middle East [2]. Most adults—65 to 90%—have some degree of lactose intolerance. It’s simply untrue that dairy has been a staple of even a plurality of advanced civilisations over history.
[1] https://medlineplus.gov/genetics/condition/lactose-intoleran...
[2] https://www.smithsonianmag.com/arts-culture/lactose-toleranc...