For me replayability comes from multiplayer as well. A PVP scenario makes the number of possible options you can encounter in a game much bigger. Let's compare a pokemon game and Showdown for example. In the Pokemon game, you play it, you finish the story (including the post game) and the game ends. There's nothing else for you to do since you've beaten everything, read everything, done everything. In Showdown however there's always another game, another opponent, another team. It will take you ages to try all possible combinations, and by then, there'll be new pokemon and items to play. Same for League of Legends or Magic: The Gathering for example. Just by having a shifting metagame you ensure that your players will never run out of things to play, and so the game is almost infinitely replayable.