Comment by lazide

The issue (for the masters, and besides any ethical issues) is being a slave master is a very tenuous position, and prone to revolts.

Too capable (but also valuable!) slaves tend to be self sufficient and strong enough to throw you off.

Too weak (and therefore non-valuable!) slaves tend to be easy to control - but are a huge drain on the system, including ‘master’ management, which is often the most constrained resource anyway in any hierarchical system.