I do, actually. Human civilization is 3-5 times older than the Romans, and human existence a hundred times older. The Romans had steel (swords and shackles), literacy, and a sophisticated transportation network. What we have today is just refinements on Roman ideas.

Welcome to HN, where 'classical antiquity' is modern.

On HN, most people think of an iPhone3 as some sort of incomprehensible antique from the Dark Ages.

The point, though, is that Roman slavery wasn't very different from modern slavery. They had all the key elements we do - the tools to both imprison humans en masse and exploit natural resources, a standardized transportation network to move both troops to where the exploitation happened and and remove goods from where the exploitation happened, literacy and bureaucracy to manage large-scale, ongoing enterprises, etc. And of course, they had their moral rationalizations for brutality, especially brutality that happened somewhere out of sight.

Without metal, without roads, without heavy transport, without structured military units, without literacy, the tools for mass enslavement really don't exist.