If market returns don't meet or exceed expectations, you end up broke on the street (and likely dead). (EDIT: Market returns have been, over the last decade or so, lower than expected; there is a school of thought that returns will no longer be as high as they were historically). [1]

That's the beauty of Social Security: it's guaranteed not by market forces, but by the existence of government. So long as the US government exists and has the ability to tax, those who rely on SS and Medicare will be provided for.

[1] https://www.theguardian.com/business/us-money-blog/2016/may/...