You have to remember that the incentive is to create a benefit for today's group, whether or not it is sustainable in the long run.
If I were a senior, I could vote in benefits that today would benefit me, but create enough budget pressure that later groups would receive benefits that are worse than what I receive now.
If I were a senior, I could vote in benefits that today would benefit me, but create enough budget pressure that later groups would receive benefits that are worse than what I receive now.