The Greeks and Romans had slaves.

True, but there's a difference between slavery as a result of military subjugation and institutionalized chattel slavery. One regards slavery as a form of punishment of one's enemies. The other regards slaves as personal property.

If a Roman enslaved a Celt, he was dispensing justice against another human being. But if a plantation owner purchased an African slave at auction, he was simply buying a thing. The slave had the same status as a plow or a mule.