Slavery was legal in most society's 2 millennia ago but most society's were not built to be depended on slavery the way it was in Sparta, Rome and the American South.

They swept it under the carpet, keeping the benefits, but keeping it a side part of their economy. Kind of like a modern country depending on unprotected labor and even child labor in the third world for its minerals or even for its Nikes.

And a lot the later 19th-20th century "englightened" countries that didn't have explicit slavery in their midst, have enslaved and exploited whole countries as colonies.