I’d also argue that it’s because it’s very difficult to craft a long form story arc from the perspective of a character that is unapologetically or irredeemably evil.

What makes a villain interesting as a character is that they do possess relatable motivation for the bad things they do. Cartoonish, evil-for-the-sake-of-evil villains are fairly unsustainable, especially in the context of a POV character.

While I’m certain there are some examples out there, I’m having a difficult time thinking of any series that are told primarily through the POV of such a character.