Much (but not all) of US economic activity occurs at publicly traded US corporations. Even tax revenues are reasonably well correlated to their success. A major exception is real estate, but if you own a home, then you are implicitly basing part of your retirement on that ownership already.

So what else would you base a retirement based upon? Empty promises from politicians whose have no incentive to deliver on them because their term ends long before these promises are fulfilled? We have that in the public sector already, that's why there's dozens of pension crises looming, from Sacramento to Boston and half the states in between. We also have that to a limited extent with Social Security, which is likewise heading for a major shortfall. And nobody really actually likes Social Security — I mean, they may be in favor of it existing, but everyone who can afford it has a private pension with a lot of holdings in the stock market, to supplement Social Security, and cover its shortcomings, and no one would trade that for more Social Security, because it would mean they end up with a lot less in the end.