Scale. It’s about scale and centralization. Sure there were slaves but capitalism made more than ever in human history.

Depends on who you ask, and when, I guess. For instance, if you asked one of the nations conquered by the Mongols, they would’ve said that empire was the largest enslaver in history.

The Mongol Empire established a massive international slave trade founded upon war captives enslaved during the Mongol conquests, which were distributed by market demand around the empire via a network of slave markets connected through the cities of the empire.

Considering this was the largest contiguous empire in human history, the exploitation and damage was pretty extensive.