Unfortunately there are too many stupid people in Congress that shot this idea down.
Things like the 08 market crash and the 1,000 point "flash crash" from last week are what keep S.S from ever being invested in the market and subject to the volatility that comes with it. Some people are smart enough to do far better than S.S would provide in the market, but most aren't.
I am not philosophically opposed to the idea of a forced savings account like that so to speak that takes all risk out of the equation so people no they are going to receive a somewhat defined amount regardless of what is going on in the market at a given time.
Congress has been borderline criminal in how they have handled the surplus, but the program itself has been possibly the most successful thing the government has ever done in what it was designed to accomplish. It brought stability for seniors in ways that never existed before it, and has always been reliable and there regardless of the economic situation in the country.
Government doesn't do alot well, but keeping S.S away from the whims of the market does more good than harm as the few who would do more with the money than S.S does are outweighed by those who would see their retirement go up in smoke through poor investment decisions. It is easy to say its their fault and tough luck, but it ends up causing political and social instability when the natives are restless.