I'll tell you exactly why. There are transitional costs associated with reform that will push up the deficit short term. If the deficit is higher it is much harder to push threw large new entitlement programs.
The AARP wants to spend the money used to fix social security on themselves with bigger prescription drug benefits and other potential goodies. They are afraid that if we fix social security there will be less money available for them to extract from the pockets of the working population.
Remember that crowds call to "fill the donut hole" on prescription drugs? What they want is to fill their pockets with cash and reform would make that harder.
My husband refuses to join, they've been sending him membership literature for the past few years. Me, I've got a couple for years to go before they'll be after me!!!