Humans treat other humans this way, too. Those with lots of power and wealth inevitably inflict such a life onto other humans. Even an individual of meager means in rich country might have some poor girl barely affording her food, yet working long hours in a textile factory, or the like. But why? Because the sweatshop job is better than the alternatives.
Babies can die if they're not held. There is some biological imperative that requires a baby to be loved. Similarly, cows have a biological imperative to live, to reproduce, to eat, and so forth. If, any any point, cows didn't prefer their life to not living, they could, as human babies do, simply not live in their environments. It happens to a lot of "wild" animals that are brought into captivity. They have problems with reproduction, with behavior, with health, and generally do not so so well out of their natural environments.
I mean, when you think about sitting around all day under incandescent lights, eating yourself to obesity, killing yourself, essentially, so that someone else can have a nice meal, or make a buck... how is it so different from the plight of most of humanity? In fact, daresay that starvation is something a farm animal might never have to worry about.
You're making the assumption that Mother Nature would be kinder or more generous to these animals, but that's not necessarily true. There are predator animals and prey animals. We are apex predators. If humanity was wiped out in an instant, you'd still have big animals eating smaller animals all day, every day. I even suggest that slaughter is a more humane way to die than being eaten alive by a predator, no?
All that aside, eating plants is more efficient, but as omnivores, our nutritional needs are better satisfied by eating some amount of meat. Sure, you can take measures to eat no meat, and I certainly have no problem with anyone doing that, I just really don't understand it.