That may be true of Plato, Aristotle, etc. But one thing I have learned from history is that there is almost always a contingent of people that do find terrible things like slavery abhorrent and were even outspoken about it. But if you are an elite, and benefit greatly from something, you are probably much less likely to be outspoken against it.
And then from inside your link, there seemed to be even more critical voices:
"Then again, the Stoics were famous for challenging common conceptions, and the founder of the school, Zeno of Citium, had declared slavery an evil in his Republic"
So now I think it is a shame, that I barely ever heard of him, despite having heard from all the others great (but slavery endorsing) greek minds. And sure, slavery was very common everywhere at that time. One more reason maybe to celebrate early free thinkers?
Most philosophers likely come from the elite classes in the past. You don't have time to sit around and think and write if you have to worry where your next meal comes from.
