Milk isn't good for you
"Wait a second," you interrupt confidently. "Don't tell me not to drink milk! Milk is necessary, especially if I want to grow up big and strong like Pop-Pop!"
Wrong. Milk isn't a great source of calcium. Ditto for vitamins D and A, except for milk that's fortified with vitamins D and A, and if you want to tumble down that slippery slope, you'll start saying things like, "Cookies are a great source of phosphorus!"
Milk isn't a great source of anything, actually, when you compare it to the nutrients you need in a day.
Most people aren't so great at digesting milk
According to pretty much any statistical model, 65% means "most." That's right: 65% of humans have a reduced ability to digest lactose after infancy. Lactose is a sugar in milk, and in those 65% of people, the body stops producing lactase (the enzyme that breaks down lactose) after weaning. Because you don't need to drink milk once you stop breastfeeding, duh.