I think the simple answer is we are mammals and feed our newborns milk and since they usually don't die from their breast feedings we equate milk to being good & nutritional. The fact is, we've discovered clay feeding vessels with nipples that contained animals milk from 2000bc near infant graves. The horn was a popular material for feeding bottles during the middle ages.

We've been giving infants and kids milk for a very long time to supplement their diets because the dairy industry was able to reach that far back in our timeline. I think we all understand that the dairy industry has been lobbying and marketing, to get a spot at the table, but so has every other commodity thats available in a global market.