Because so few people actually redeem their FFMs. They hoard them up. And then quite often, completely in the terms of the program, they don't earn enough and their balance gets wiped.
And yes, reward flights usually require you to pay tax, and for popular routes there may be only one FFM redeemable ticket per class per flight. Meaning you have to book ages in advance, choose less popular routes/times or be damn damn lucky.
And yes, reward flights usually require you to pay tax, and for popular routes there may be only one FFM redeemable ticket per class per flight. Meaning you have to book ages in advance, choose less popular routes/times or be damn damn lucky.