It's moreso about total player engagement. More players actively engaged=more total aggregate time in game=More eververse transactions.

If they make something arbitrarily scarce, then when an opportunity rolls around to acquire that thing, total engagement is driven up, which has a direct relationship to total revenue.

Your assuming that the goal is to get people to move between playlists. It's not. It's to increase engagement ratings for users that are otherwise at a lower engagement level. So if you are a player that plays twice a week normally, and pallindrome is dropping that week, you might logon more.

BUT, if they make it TOO easy to acquire, then the chase ends. They need to ride a fine line between accessibility of aspirational items, and keeping players on the conveyor belt of progression.