The other question is why do we never see any more than a Sith master and his apprentice? Are there really only one, two , maybe three people practicing the ways of the Sith at one time? Where are the rest of the Sith at?
I kind of see it like monarchy. There’s legally only ever one King or Queen, but obviously history does not play out that cleanly. Civil wars have often been fought over whose is the true royal lineage.
It’s the same with the Sith. There’s supposed to be only ever two at once, in a continuous lineage. But there can be multiple claimants saying they’re the two true Sith and the other guys are pretenders. This happens in the Clone Wars cartoon, with Maul & Savage and Palpatine & Dooku. Palpatine claims that Dooku is the apprentice, while Maul claims that he is still the apprentice and that Savage will become the Sith Apprentice when he kills Palpatine and becomes the Sith Master.