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.

I don't know about your country, but in my country whenever there is a power outage there's news where some 10000 or 80000 chickens died because of the power outage

https://www.facebook.com/nhnoticiasmanoelribas/posts/queda-d... 5 days ago, 20k chickens dead in just twenty minutes without power

But power outages don't cause chicken death, at least not directly. The most immediate cause of death is dehydration. And it happens because chickens are kept in an environment so confined, so absurdly cramped, that without giant fans blowing 24/7 they overheat, dehydrate, and very quickly die. (in some cases, the beaks are clamped, too, so they don't peck each other to death)

That's what it takes to have cheap chicken and cheap eggs. That's what happens when we are so detached from animal food production that it becomes a commodity. It doesn't matter how much the animal suffer, as long as the consumer can ignore it safely when they eat. And the reason this can happen is that animal well being isn't worth much. (veganism is just the position that animal well being is worth a lot. It isn't merely a diet choice and has far reaching implications. For example, if you are vegan you ought to be against the destruction of natural habitats, fossil fuels, etc)

Btw this "environment" I described where chickens are raised in hell doesn't look a lot like the natural environments the chickens and other dinosaurs evolved in during millions of years