The past 4 billion years of life for prey animals has been "get born, eat, get eaten by a predator." They have never experienced any other environment. Why do we owe them a different one?
For me the issue isn't with the killing/eating of animals. Rather, it's how they are treated during their lifetime by the meat industry - which is essentially optimizing for the minimum conditions that can still provide meat that can be sold legally. I'm not a vegan by the way, but I can appreciate the moral case vegans make.
For the same reason that we now consider murder, assault and other actions that harm people morally wrong. These have also been a part of life ever since humans or other hominids roamed the earth, we just determined that they are morally wrong later on.
Oh? Are you going to do a citizen's arrest on a wolf for traumatically murdering a deer, thereby violating its right to avoid cruel and unusual punishment?
The past 4 billion years of life for prey animals has been "get born, eat, get eaten by a predator." They have never experienced any other environment. Why do we owe them a different one?