This question really feels like it's meant to be a homework question, but I'll give a brief answer for Rome.

Rome depended hugely on slaves at every level of their society, from the salt mines to the palace.

The 'why' is going to be nearly impossible to answer. There were different societal expectations, different value placed on human life, warrior culture, because drudgery sucks, lack of mass media and so on.

"Because they could" might be the closest you're going to get to an actual answer.

Rome also relied heavily on slaves for education. They tended to use Greeks for educating upper class Romans, and Romans also used Greek slaves for the purpose of teaching Greek to Romans. Highbrow Romans loved Greek literature and Rome imitated the Greeks for many things from theatre to art.

Roman slaves could also spend a long time associated with a family, or they could be freed after spending a short time in slavery.