Slavery was a step up in civilization from pure savagery.
Another option seen in American Tribes: adoption into the tribe.
It was sometimes offered to valiant warriors.
Many tribes "adopted" as many children as they could get, because they often had trouble raising enough to keep up their population.
All over the world, pre-pubesent boys and all fertile women were valued, even without slavery.
In the case of at least one civilization from which Europeans obtained slaves, the king sold his own ‘surplus’ people into slavery, probably those from families he considered potential rivals.