Programs have to devalue: at a constant number of points/$, as the average spend goes up (with inflation), the points required for redemption must rise. Otherwise, what started out as (e.g.) a 10% (on average) rebate, will rise to 20%, 50%, . . .
They could slowly decrease point earnings (e.g. from 5 points/$ to 4.5 points/$, to 4 points/$, etc.) but that favors people who have stayed with them in the past rather than those who will stay with them, and gives people less incentive to "use up" points now, leaving the liability on the books.
For that matter, increasing the redemption prices looks like a lump of profit.