I shouldn’t have used the word modern here, but I’m inclined to accept that there are some universal principles that rationally make sense, like not enslaving other people against their will or abolishing hierarchies.

The exact opposite of your proposed principles can just as easily be proposed to 'rationally make sense', though.

It's not even that I disagree with those principles. I agree. It's just that they're very much a product of our modern society and our upbringing in it.

This makes even more sense, if you assume that sympathy is a basic human function. To break it down to a personal level, it makes even less sense to keep slaves, for example, if you are able to feel with them.

Let me lay out a simplified, non-Roman scenario for you.

Your sedentary tribe has been feuding with another sedentary tribe for a very long time. Hatreds run deep, all that jazz, generations-long. They don't speak your language, they don't follow your gods or ways, they don't share your moral values. But by a stroke of pure luck, you are in charge when the rival tribe is defeated, totally. There is disagreement within your tribe over what to do, so you are the one who must make the final decision. Do you:

A. Attempt to integrate them into your tribe without oversight despite the long-lasting enmity on both sides and the knowledge that the murders will begin again almost immediately without any robust cultural, philosophical, or institutional method of integration and reconciliation, and that said conflict will weaken your tribe against others, potentially leading to you and all of your friends and family being murdered by others?

B. Expel them, despite knowing that they have many other enemies in the region and there is nowhere for them to flee to, meaning condemnation to death by starvation or else by other hostile tribes?

C. Let them return to their village as though nothing happened, even though this will not end the conflict and very well may result in the total reversal of your fortunes and the death or enslavement of your own friends and family?

D. Attempt to integrate them into your tribe with significant oversight, even though this establishes a hierarchy by which 'your' tribesmen are given power of 'them'?

If you read historical accounts of slavery before the trans-Atlantic slave trade, the common notion of slaves as conceptual enemies or foreigners, even when born into the condition, is not a coincidence.

You are a tribe that has recently begun a labor-intensive agricultural form of existance. A tribe the next valley over sees your farms and raids your village. They kill your warriors, rape your women, and streal your food.

The next time they come back, you are ready, and you kill a bunch of them. Most flee, but a few are too wounded to run away.

What do you do with these people?

1. Let them hobble away/escourt them back to their tribe, where they will presumably regain their health and then raid your village again.
2. Kill them and be done with it.
3. Restrain them so they can't raid you anymore but also so you don't have to kill them, as they are begging for their lives.

If you choose 3, then what? You have some kind of cage or shackles for them, and they do nothing all day while they are brought food and water? No one in the village will stand for that while they are going out and hunting, foraging, and farming. So what do you do with your prisoner? You make them work - probably doing the shittiest jobs - in order to earn their keep instead of getting their head smashed with a rock.

Hence, slavery. Then, do it for several thousand years, and it is part of your culture.

Some days a man is just tired of people pretending that specific moral codes are an innate attribute of human beings instead of a complex construct and product of our societies.

... most days a man is just tired, period.