>>14284097>>14282077This conversation is already going on but your question is better worded.
The characters, the story, the world the music and the history and mythology it's all pulled from.
The multitude of interactions and relationships between the characters, imagined or otherwise, their attitudes despite the gravity of their respective situations, the hypothetical scenarios, the implied horror rooted in the nature of many characters, and how all of it builds and builds upon itself. As the games come out and more of the world is revealed, each character's place in it feels more and more storied, more rooted. Everything they did and everything they are shares a space with the new but their existences are never trivialized the way faceless or characterless adversaries are with the passing of time.
It really doesn't just come down to one thing, for me at least.