They're not part of negotiations because there was such a backlash during the Reagan era. Part of his campaign platform was to reel in the entitlements. Then he discovered that people like their entitlements. George Bush tried to privatize Social Security, and the idea went nowhere in a hurry.

The value of Social Security is the expectation that it will be bailed out by the government if it goes bust, for instance if the trust fund is depleted.