You think of US as a monolithic entity and that really isn't true. There are lots of agencies with lots of employees and future obligations; they all have their own budgets and "books"; congress doesn't like bailing out an agency due to them thinking they have the whole US budget to spend. Maybe if we bothered to just force all public employees into 401Ks, we could summarize it as mostly SSI and Medicare future needs (though we are increasingly hitting pay as you go).