(I listened to this while writing this!)
After re-familiarizing myself with Old School RuneScape (OSRS for short) this week, I think I can really state why I think the game is so prevalent amongst the people who play it (including myself), to the point where people in the community will often jokingly say that "You don't quit RuneScape, you just take an extended break from it." Sure, some of it is certainly nostalgia from a time gone, especially since OSRS is specifically made to strike that nostalgic chord with how it looks, sounds, and feels; but I think that the true reason lies a bit deeper inside, it's the absolute freedom the game affords you that makes people want to come back.