2016 Postmortem

I'm fairly old, 50+.

If I were to put pen to paper and calculate the amount of money I've paid into social security so far, I'm thinking it would be a staggering amount of money. If the government came to me with my pile of money and told me I could not touch it until I was 65 but I did have control over where it was invested, I would really like that.

I could choose treasury bonds (in effect the same place social security is invested in right now)
I could choose the American stock market, the international stock market, precious metals, commodities. Obviously these would be heavily regulated and diversified mutual funds and it is not like stock-picking.

Even if I didn't understand anything I could just leave it in T-bills and, in effect, social security stays public for me.

Then I think that if I was just starting out (25 years old) I would really, really like this.

I'm not sure why giving me this flexibility is bad? Could somebody explain this to me.