Livestock are not things, either, but the management of them does absolutely fall into the "they're not humans, either" category. Anthropomorphizing is a HUGE PROBLEM when dealing with animals in general, but with livestock especially purely due to the numbers produced.
The thing I didn't mention, and something that people rarely consider from this perspective, is that nature culls. In the wild, FTT animals will suffer and die slowly. They might be lucky enough to be caught by a predator and ended violently but swiftly. Animals not suited for their environment will suffer and die off at some point. Sick animals will suffer until their body gives out and they die. Injured animals will suffer, often rotting on their feet, before succumbing to the injury. If they make it, they may recover fully or they may be lame in a way that causes them to suffer hunger or pain for the rest of their lives with no way to abate it. It will probably mean their (slow, painful) death at some point. Predators may assist with swifter but violent ends. Even healthy animals may die to accidents, predators, etc. Survival of the fittest and all that. This is natural, sure, but also horrific. The two are not mutually exclusive. And it's necessary, for this balance to exist, or animals would overwhelm their environments and none of them would get to live.
In an artificial environment (ie, domestically-kept animals), humans are acting as nature, and I feel it cannot be stated enough that we are far far far FAR kinder about it all than nature will ever be. Well-kept domestic animals experience a life of luxury that their wild ancestors could never hope to have dreamed of. They get rich foods, endless clean water, a safe place to live and sleep, health care as needed, and a swift, gentle end. Their progeny will produce progeny which will produce progeny which will produce progeny, into time everlasting as far as they're concerned. For creatures whose goal in life is to pass on their genes, that's a pretty damn successful life
But the other side of that is that the domestic animals (as a whole) ARE wildly more successful at reproducing, and those offspring are wildly more successful at surviving, which means there are far more of them than would exist in nature. Humans must act as nature in this case; and this means that responsible keeping involves pruning the animals not suited for the environment they are in (in order to make animals that do not suffer in this environment), gently putting to rest the animals that are suffering (particularly in ways that cannot be healed), and, yes, pruning the excess to feed ourselves, in return.
Humans keeping domestic animals is mutualism. They gain a comfy, stress-free life, success in passing on their genes, and a kind end, and we get food (or company, or in some cases, work). The question "why do we do this to animals" seems a lot sillier a question when you actually consider the animals as the animals they are and what "nature" actually means, and look at what good, responsible breeders are doing, and realize that they usually aren't doing anything "to" animals, but rather "with" them.