Cows, like dogs and woolly sheep, are a product of domestication. That is to say, they didn’t exist in their present form in the wild. People took their ancestors in and selectively bred them for traits they wanted – such as sheep that make a lot of wool, and cows that make a lot of milk.
As such, it’s kind of useless to ask what these animals did in the wild before they were domesticated. They’re a human creation. And since we did this to them – created cows who have to be milked, and sheep that need to be shorn, or else things go poorly for them – it is pretty much our responsibility to take care of them. Like I said above – we “broke” it (created them to have different needs from their wild ancestors), we “bought” it (responsibility).