In general slaves are hard/expensive enough to get that you keep them around. You need to go to war (which can kill your people, or even cause you to lose and become the salve), or buy them from someone else who did that work. Plus you need to teach them to do their work, which means a fair amount of effort if you are just going to starve them. (though this is also why slaves generally were basic manual labor and anything highly skilled).

Though you are correct, there have been various concerns for the welfare of the slaves over history, and cost of getting more is a major factor into that.