Reasons I can see, outside of "objective morality" that morality even exists.

+Without it society wouldn't work.

-You can't trust a person that would kill you

-You can't trust a society that will off you if you ever end up disabled

-You can't trust a society that will be fine if you are killed for another's gain

-A society works best when people respect and do not actively get in each other's way

-A society works best when all it's members are in a healthy mental, emotional, and physical state.

Animals are not part of society. I don't care how smart they are, until they are smart enough to respect property rights or get a job, I will feel zero remorse for eating them.

Animals that are peoples pets are a part of society. I do care about my pets, and it will hurt me if you hurt my pets. If you eat an animal that is commonly a pet, you will run a high risk of eating someone's pet. Secondly, sometimes people just don't like doing things because culture. Many countries eat dogs, cats, horses, and some treat cows as pets.

Abuse of an animal causes a) that animal to remain alive and fearful of humans, it may very well go on to kill someone. I'm sure there are stories of dogs getting out and killing after mistreatment. B) abuse of an animal reflects on your personality. I do not want to share a house, a street, or a mcdonalds with someone who will hurt to make themselves feel better.

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So there it is. "Why is abuse of animals bad when we eat them!", "Why is it moral to kill pigs that are smarter than dogs", "what about disabled people, why not eat them". All of those questions are misleading and based on a kinder-gardeners understanding of morals. "hurting things is bad" "you should share" "no tattling".

And if we find aliens that are delicious, or aliens find us delicious... Well, we better hope we/they have comparable societies. Otherwise we/they just found a new source of food and/or war.

(also any intelligent species would wage war, making eating their meat not worth it at all.)