> And yes, slaves could be freed but that was also true in the American slave era. Just because something is possible doesn't mean it's accessible, particularly by the slave themselves. Also, freedmen in Rome had less rights than citizens.

In the US slaves were used as collateral for loans. In a way slaves weren't owned by plantation owners, they were owned by the banks. That if anything explains some of the pernicious aspects of US slavery. Also likely the US banking system. And the mistreatment of blacks after the civil war.