Because it’s speciesism. If we started giving birth to humans to eat them, that would be absolutely outrageous, but to do that to animals seems perfectly fine to most people. Animals have the same desire as we do not to be killed or abused, and to live a happy life.

This argument also implies that “dominionism” is wrong, i.e. all life has a right to not be killed or abused. Yet human life is impossible without killing and consuming, other living organisms, be it plants, animals og fungi. Thus it is unethical to continue living.

This argument is bad, because for human life to be possible, you must draw the line between life that you consider ethical to kill and life that you consider unethical to kill.

It’s not about “all life” but about “all sentient life”. Only beings that are able have pleasant and unpleasant experience should be considered. If something (living or not) cannot experience suffering then you can’t harm it, by definition.

Sentience is studied scientifically. It cannot be stated with absolute certainty but scientists have good sets of criteria and experiences that helps identify it. With the current knowledge it’s almost certain that all mammals are sentient, like us. Fishes and birds are also very likely to be sentient. Some species of insects are probably sentient while others may not be. And plants are likely not sentient.

But even if all living things are sentient, it doesn’t change very much. Speciesism means treating beings differently only because they belong to some specific species. There are good reasons to treat different beings differently but they should be based on the beings’ interests, not their species (and studying sentience helps identifying these interests). It’s very likely that we do less harm by growing plants than by breeding animals. And even if it was the same amount of suffering we would still do less harm by avoiding eating animals because breeding them to eat them actually requires more plants than just eating plants. We should seek to minimise suffering and avoiding eating animal is a good way to do that.

why should sentience matter at all?

Because if something is not sentient it cannot have negative experiences, so you can’t be harmed.

so?

The question was “why is eating meat bad?”, my answer is something like “because to have meat you must harm animals”, and someone answered that “we always harm something when we eat” and my answer is “no, there are foods that you can’t harm because they are not sentient”.

first, you can’t prove plants aren’t sentient. second even if you could, why should sentience matter? what ethical system even accounts for sentience as a factor of right behavior?

Even if plants were sentient, and I’m not saying they are, but if. Would you rather “kill” orders of magnitude more plants to feed them to animals, then kill the animals and eat them, or would you kill the plants and eat them directly? One of them causes a lot less harm (if any at all), and it’s not eating the animals.

well, first, animals are mostly fed plants or parts of plants that people can’t or won’t eat, so the scale of the difference you described is orders of magnitude less than you are suggesting.

but, more importantly, why should sentience matter?

finally, whether i buy food from a shelf or not, the creature (flora or fauna) it came from is already harmed, and my purchase causes no more harm to it, so eating it has exactly no impact.

Those plants still have to be cultivated. If there are no animals to feed those plants to (for instance, low quality corn or low quality soy), the lane can be used for cultivating food for humans or in the case of low quality soy, the rain forest doesn’t have to be mowed down for it. Sentience matters because ideally, one should strive to reduce harm as much as possible. Especially unnecessary harm. There is a reason why I don’t torture cats and dogs for fun, and it’s the same reason I don’t eat killed and tortured cows, pigs, chickens, etc. just because I like the flavour of them. And of course your purchasing behaviour has impact on the amount of harm caused. Maybe not instantaneously, because it is indeed on the shelves already, but just like with voting in elections, if you don’t buy products that cause harm, demand drops ever so slightly. Then when more people inevitably follow, demand drops further in a big enough quantity to matter. That’s why you see a lot more vegetarian or vegan options in your supermarket today: because people buy them.

the soy fed to livestock is almost entirely the industrial waste from making soybean oil.