When people get old, they can no longer physically do the same things, or perform at the same level.
Some even get very disabilitating diseases, such as Parkinsons, or Cancer, or Alzheimers, or Strokes. These things are very common. My Father was healthy as a horse and then wilted away with cancer at 72 (young?).
You cannot possibly expect older people to just continue their careers as if they are 30-40 years old forever, and their Employers will lay them off at the snap of a finger, once they become unproductive.
Besides, Retirement is one of the attributes of the whole American dream in the first place. We are not slaves here, nor should we be. The Quality of Life in this Country being ripped away from us. Why do we accept this?
Life is about a lot more than just working like a rat in a cage until we drop dead. If you wish to do that, that's fine. But not everyone can. Many people simply physically cannot.