@sun 1. It wasn't backbreaking, there was actually a living history thing where some farmers matched the output of the average slave manually picking cotton and they found out slaves didn't work any harder than a white farmer family in the modern day.
2. They were fed, clothed and got a ticket to the new world, whereas otherwise they'd've been slaves in Africa. We can tell because the supply of slaves was relatively inelastic for hopefully obvious reasons.
3. Slavery was the normal state of being for a large proportion of any population in history and the only reason you see it as monstrously evil is because you're looking at it through the lens of modern morality. The life of a slave wasn't all that bad.