There was an Outer Limits episode that addressed the slavery thing. Aliens came to enslave a group of humans, and one of the humans went so far as to point out that this was silly: Any civilization that can build starships has no need of human slaves.
The alien explained it very simply: Their culture considers the use of mechanical labor 'demeaning.' Presumably slaves serve for them as a form of status symbol: Anyone can afford a robot, but having slaves to tend to their needs is the mark of true wealth.
