kronix, CelticMutt, or a simpler answer. They know history better and understand the reasons and consiquences of this form of slavery. It isn't like any of these worlds have had a racial based slavery system. It has almost always been: criminals, vagrants, the poor, the homeless, the orphaned, or those who were captured in war... i.e. slavery because its better than simply killing everyone and they have neither the money nor the resources (primarily food, but security as well) to have systems that we have now (not that WE utilize or impliment thise systems very well, but I am tired of getting into that...)
Slavery only continued to exist in one place due to racial biases, America. Everywhere else, while having it for a short time, started with: "what do we do with all of these people?"
(That doesn't make it good, but also gives a reason for the main characters to not simply run around crusading against the only system these ancient/elder people have for crowed control. Their actions would literally be pointless if they tried to simply free those in slavery... [Prime example, US even decades, a century after the civil war.] Now, if they could provide jobs for the people they free, without taking jobs from those already free, that is another thing.)
I mean, even for those MCs who do immediately free the person they bought... the world looks at them and their action and says: "yeah, that's what you think. Let him/her get far enough away from you and they will be captured and sold again." Without a source of income, or a way to get said income (stable job) there is no way a freed person in these worlds would remain such. Much less in those where the individual is cited by their CLASS (or mark).
Only those MCs who create lands or nations of their own could go around ending slavery, and only by importing the labor by buying the people in other countries and freeing them in their own. This would never end, though, without elevating the other countries to a level of technology and... well... civilization where slavery is no longer needed, and then encourage policy to remove it.