Weeeeeeeeeeeeell...less so because of sheer arrogance, more to do with EU writers dragging them out of the grave kicking and screaming. Keep in mind, EU continuity originally stated that the Sith were done and dusted, and completely wiped off the map. The closest anyone got to a proper Sith revival was the whole Scepter of Ragnos arc in Jedi Academy, or Exar Kun's spirit in the Jedi Academy trilogy.
Huh, never ntoiced that before. But yeah, Dark/Fallen Jedi and Sith spirits were the standard enemy for ages. The Sith were, for a long time, simply not allowed post Galactic Civil War.
That is completely true, right up until we get to the introduction of Korriban in what I believe was either KOTOR 1 or a novel just before, but it was in the early 2000s, so right around the height of the prequel trilogy. Ancient birthplace of the Sith. Once you brought that into the universe, anyone could become a Sith by studying the ancient teachings. Then Ziost came along, and Dromund Kaas (introduced WAAAY before SWTOR). But it all began with Korriban.
True they could never truly be "Sith" in the same sense Darth Sidious or Darth Malgus or Darth Bane was, but as we all know, Exar Kun became a Sith Lord through a simmilar method: Studying ancient Sith teachings and influence from long dead Sith spirits. And yet nobody disputes if HE was ever truly a Sith.
EDIT: I was just going for an IN-Universe explanation.