> Prior to the steam engine, what sources of energy you have?

Oxen? Paid laborers? It's not like the American South was unique in needing farm workers.

> Basically all the settled civilizations used some sort of non-free or at best semi-free labour.

The South was notable in clinging to slavery long after it had been abolished elsewhere.

> And given that humans are very good at rationalizing away their conditions, the cultures adapted to being comfortable with it, even considering the societal inequality as something ordained by the gods or karma.

Good, then we agree; it was at least in part cultural.