Why would you assume drinking milk from a different order of mammal is good for you? And why would you assume that because we have been consuming an item for millennia that it is good for you?
That's literally how you could assume it's good for you but there are also plenty of studies that support the assumption. Plenty show that low-fat or non-fat yogurt and milk are "good" for you.
Humans have been milking everything they can get their hands on since prehistoric times. There are a number of reasons to not eat/drink those things (taste, vegan, intolerance, etc) but that doesn't suddenly make yogurt, milk, and cheese "bad" for you.