Plenty of people go out the door after X years then go back in worse states than when they first showed up. Once picking up a drink/drug/whatever, they're off and running again. 10 - 12 are "maintenance" to stave that sort of thing off. There's no end to working 10 - 12. They're a daily practice of continued growth. There's no end point (recovered), recovery continues on (recovering).

It's also a practice to keep everyone on the same level. Everyone is an alcoholic -- otherwise it'd just be a bunch of old farts telling new guys what to do (then hardly anyone would come back).