The better question would be where people got the ridiculous notion that mages would somehow be incapable of creating any other kind of system.
If non-mages make slave-based systems, and we know that they do, then discriminating against mages on that score is absurdity of the highest level.
Making examples of IRL countries is irrelevant to DA. Thedas isn't our world.
So far we saw normal countries not based on slavery, but we didn't sede a magocracy country without slavery. Dalish and Chasind are clans/tribes, and In Rivain (other than we know too little to judge it), mages have power, but they're not the only one to hold it, so it woudn't be a real magocracy. We don't have an example of a magocracy country not based on slavery.
I wouldn't say that a magocracy country has to run on a slavery system, but that's just theory.