Look at the flip side of the equation. If scores were reset every day, would you bother, knowing that in a day, it'll be as if you never played? Now extend it to a month. Then extend it to 20 months.
Replayability also requires that the activity contains value (more specifically, it has to confer more value than other activities, or else you'd just go do that other activity). Why do some games include unlocks? To give the player value replaying the game getting those unlocks. What happens when they get them all? They stop playing because the value is gone (there is still some value from actually playing the game, but this rapidly decreases from reptition, and can even become negative when it becomes more work than fun)
Once again, the problem is *not* the devaluation. It is the devaluation to NOTHINGNESS. Devaluation increases replayability for those who want to stay at the top of the rankings. But devaluating it to zero wipes out long-term value.
Look at it this way, we have a player who plays for 10 months, and then stops playing after that. Let's say later on, a player of equal skill (gets around the same score every month as the first player), plays for 5 months. During those 5 months, he'll have the satisfaction of being higher in ranking than the older player because his scores are fresher. However, if he stops playing as well, many months later, the first player will have the satisfaction of having a higher ranking than the newer player because both their scores have devalued. See how BOTH players gain something?
If instead, all scores go to 0, then they'll both be ranked the same, and only the newer player gained satisfaction relative to the older player.
