Because they suck.
But seriously, I don’t talk about Air Miles because they’re very frustrating to redeem for flights, access extremely limited award inventory, and carry high surcharges. If you’re going to a store that offers Air Miles, you might as well collect them, as you’re getting a marginal return, but don’t expect to use them for air travel.
Unlike Aeroplan or other frequent flyer programs, the best redemption value for Air Miles is gift cards (I like the VIA Rail gift card). You used to be able to redeem Air Miles for movie passes for good value, but they recently devalued that redemption. I’ve also heard you can get good value for redeeming on Vancouver Aquarium tickets.
Do I collect Air Miles? Yeah, when I do groceries or get my favourite gin. However, I hear time and time again people telling me they’ve collected Air Miles for years, and when they went to redeem, are unable to. Air Miles has set up an expectation that they’re meant to be used for air travel, which I disagree with. If you collect for a long time, can stomach the fees, and you’re lucky with the available award inventory, you can use them for air travel (many years ago, I redeemed a flight to Baltimore… which in hindsight, I should have just paid for, as the surcharges made the redemption cost close to a paid ticket). Reset your expectations: Air Miles are optimally for merchandise/gift cards/money-off redemptions, and not for air travel.