The reason that slavery was so common around the world is that after a conflict is won the winners have to do something with the defeated.
They can’t just walk away- the defeated will lick their wounds and seek revenge later, as any Scotsman could tell you. And you couldn’t take them with you because you’d have to sleep with one eye open all the time just in case.
The usual method to deal with the problem was to slay everyone in your opponent’s tribe, if you could. Later, if your tribe was strong, you could spare some of the defeated. You’d still have to kill all the men and older boys left after a battle because if you didn’t you would have to fear they would seek revenge. You could either leave the old and feeble to poverty and starvation, or slay them too, even justify it as mercy. Healthy women which could be of use, and small children who were no real threat with no families to run back to, could be taken as wives or be traded among the victors’ people and allies, and the memory of the defeated tribe would fade.
Small tribes living a subsistence lifestyle simply couldn’t afford to take many captives, and without metal to make shackles, or the means to make impenetrable fences or jails, POWs were out of the question. Adopting was an option for the very young, but only so many adults or older children could be taken in before their numbers would dilute your people’s strength, so killing was the only practical means of dealing with the defeated.
When tribes grew bigger they could afford to spare more captives, even males. If they discovered they could sell the surplus then killing them was a wasteful option, while selling slaves was ‘more humane’ than the alternatives and profitable.
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.