Yeah for most people, it's being competitive that has kept this game alive.
Yes, including myself, many people simply love the game for fun.
But even when you playing for fun, you are interacting with other people, you laugh at funny moments, or gaze in awe at someones skill or something cool you just did, challenges are ok but they've seen far better days of activity, just hearing about Ryan/LordHound and their old friendship/rivalry, it's amazing because I know that feeling of sitting for hours trying to get 1-UP on your buddy, just to say, it's your turn now: