Miles don't expire, but they make a calculated estimate about how many miles will likely never be redeemed for accounting purposes (breakage).

"To value the miles earned, we consider the quantitative value a passenger receives by redeeming miles for a ticket rather than paying cash, which is referred to as equivalent ticket value ("ETV"). Our estimate of ETV is adjusted for miles that are not likely to be redeemed ("breakage"). We use statistical models to estimate breakage based on historical redemption patterns."