As has been mentioned earlier, the problem is the pensions that are around today were based on the assumption that people would live for a handful of years after retiring.
Now the average person is living around 20 years and some even more. Some for considerably longer than they actually worked for.
As a result, over their working lives, this generation didn't pay enough in taxes for this unforseen increase or for the massive increase in medical costs that comes with it.
Additionally they hold so much voter power due to better turnout and just big numbers generally that politicians won't reduce the pensions.