Because they can and they need money since they refuse to fix the current fare system which is broken.
I went DFW - ORD - NRT - SIN for $ 651.80 20,800 miles last weekend on AA and today priced DFW to Kansas City at $ 735.00 returning in 2 days 922 miles. I understand why the business traveller is tired of this nonsense.
The airlines really do not care if you ever use miles for tickets as they prefer cash
People talk about liability of the airlines regarding miles outstanding, but if they are not used, it is like Third World debt which will never be repaid.
Every month the changes to programs make the miles worth less and less and they make all the rules on how we use the miles.
Those of you who are carrying 1,000,000 miles in accounts planning to use these 3 - 5 years down the road are likely to be surprised by what it will take to use award tickets including fees.
Plan ahead and use them every chance you get if it makes sense.