We had slavery for thousands of years because the enlightenment occurred only a few hundred years ago and it brought about the concept of liberalism (not like liberal/conservative but liber like short for liberty or liberate - meaning freedom). Up until that point there was only basic pathos that would allow people to feel bad for a slave's conditions but usually not to the extent that it would lead to a full abolition movement.

There have always been societies that didn't have slavery, they have happened on every continent (except that one) and in every sufficiently long era.

Are you a vegan by chance?

I feel like that's the next big moral shift. People lionize dogs and cats, and harming one makes you literally Hitler. But there's not a lick of difference between a dog and a cow.

I think that an objective ethicist would absolutely say veganism is the only moral choice, and that anyone who isn't a vegan is knowingly participating in unimaginable cruelty.

But in our current context, only a small fraction of people care. Including a lot of people who look down on people of the past for not being as amazingly moral as they are.