What makes an RPG addictive? The same thing that makes anything addictive: the repeatedly broken promise of satisfaction. If it actually satisfies, then it’s not addictive.
A satisfying game makes you happy, and you stay happy when your session is over and you walk away from it. Sometimes you can even stop playing early because you already got what you wanted from the game.
An addictive game makes you miserable, but you constantly feel like it could make you feel better if you play just a little longer. You don’t want to walk away because whatever psychological need drove you to play the game is still unmet, but it will remain unmet as long as you play the game.