West Africa has had slavery since before recorded history into the present day. USA slavery was an explicit copy of West African traditions, at least at first.

Slavery is economically quite harmful, yes, and Leftists willfully ignore this, yes, but there are non-economic reasons for its persistence.

(a) Domination of other humans is wanted by many people

(b) Fearful owners of potentially rebellious slaves love and appreciate a powerful State that promises to enforce their "rights" at other taxpayer's expense in blood and treasure.

Classic "coordination problem" - the costs are widely spread and borne by free workers, taxpayers, and security forces; the benefits (emotional, sexual and financial) are concentrated and accrue to a narrow elite who often all know each other.