Because we would rather keep more of our money and choose to make our own decisions regarding saving for retirement. And the current system cannot sustain itself. Even supports of it will admit that.
(How did defense spending come into the conversation?)
I brought it up. There are really only two things that are worth 600B to us, apparently, and I wanted to lump them together for the purpose of my diatribe. It should be noted that the the current system cannot sustain itself in both of those 600 Billion-dollar outlays.
Also, I agree with your succinct explanation above about privatization or partial privatization.