That has two causes that I am aware of. First, some people are genuinely lactose intolerant due to a gene variant in some ethinic groups. Second, pastuerization. The heat destroys lactase, the enzyme that the body uses to help digest dairy.

Pasteurized milk is a big factor. Raw milk contains every enzym that's needed to digest it well. iirc nostr:nprofile1qqs0ulmtcmmn8zmkh0uqmdqz4hn9j5lzpvhj8enw3xpqfd3ucsjnngcpramhxue69uhhyetvv9ujumn0v3jhyatwdejhyuewdejhgam0wf4szrthwden5te0dehhxtnvdakqz8mhwden5te0dehhxarj9ejkjmn4dej85ampdeaxjeewwdcxzcm9fma2ah did a video about that topic