We eat cow. I see no difference between drinking cow’s milk or say, eating a chicken’s egg.
Entire human populations are thus "lactose intolerant." Over the years, adults from other populations develop or adapt to a level of tolerance for cow's milk, but heavy cow's milk drinking by humans is not good idea because of immune system reactions to cows' milk proteins, which occur in all humans, some at a barely perceptible level, others much more seriously. For example, a child may be intolerant of cow's milk, but thrive on goat milk, or sheep's milk, even horse milk!
There are also benefits to humans, especially in the provision of calcium, balanced with an individual's lactose sensitivity. Some people are also violently allergic to eggs, an otherwise great source of protein for many others.