There just aren't enough pastures to raise enough cows that way to meet demand. Otherwise, it would have been done already, its much cheaper to let cows roam around eating free food, instead of having to pay for it.

The reason why factory farms exist is to meet the increasing demand. Farmers have every economic incentive to try to raise the animals in the cheapest way possible.

Still the cow is a prisioner, it has its childs taken away at birth and shot and gets impregnated immediately after, to keep producing more milk.

If the cow stops producing as much milk, it gets shot as well. Even if the cows lived a live chilling in the fields, all of this would still happen.