[deltarholamda]

>why single out the debt Social Security owns for that treatment

Because it's the biggest slice of the federal budget pie, and currently it goes to the segment of the US population with the highest average net worth.

Whether you think it's right or wrong, it is a massive chunk of mandatory spending. Combined with Medicare, it's close to 40% of the entire budget, and it is ultimately unsustainable.

[walkabout]

> Because it's the biggest slice of the federal budget pie

It's not spent out of the general fund. The only—only—effect Social Security spending has on the budget is that for part of its spending, it calls in debt the general budget already owes it when it spend its money (which is invested in government debt). That money came from a dedicated, separate source, and wasn't generated by deficit spending, but by actual income. That money bought US government debt, so we owe that back, and that repayment is the only part of any of this that affects the budget.

[EDIT] To boil this down for international readers or people who just haven't ever bothered to learn the super interesting (LOL) way Social Security is structured:

1) Social Security is funded by the majority of the money that comes out of workers' paychecks under the "FICA" heading (some goes to other benefits-related stuff, but most of it goes to Social Security). This is a static proportion of ~15%, not progressively bracketed like ordinary income tax. About half is paid separately by your employer, typically, but people filing as independent contractors (form 1099) pay the whole shebang. This tax also phases out abruptly at a certain level (I forget exactly where, something like $200k for an individual tax filer I think) which means it's actually not just not-progressive, but rather extremely regressive (i.e. it preferentially targets income dollars with the highest marginal utility, not the lowest)

2) Under certain circumstances, but mostly when you get sufficiently old, you can start to draw benefits (payments back to you) out of Social Security. These benefits are (basically) adjusted for how much you paid in. Paid in more? Get more back.

3) For a long time Social Security ran a surplus from their FICA income, so that money went into the Social Security "Trust Fund". This money is not available to the general budget. The law is structured such that the excess money couldn't just be spent on other stuff.

4) ... however, you don't really want money just sitting around inflating away, you want to invest it. Plus, sure would be nice for the rest of the budget to access that big tempting pile of money somehow, if you're a politician and kinda an asshole (but I repeat myself). So, the trust fund bought government debt.

5) Social Security is now deficit spending, because we have a fuckload of old people drawing benefits from it, and not enough young people funding it to balance that out. That means it's spending down the trust fund (ITS OWN MONEY that it ALREADY COLLECTED). This requires cashing in some of those IOUs.

6) "Well when it runs out of money it saved up, won't that mean that spending becomes deficit spending against the general budget?" Nope! Not without changing the law, anyway (which opens up practically infinite possibilities, so it's kinda pointless to introduce to a discussion of "how does this work?"). What'll happen is the SSA (Social Security Administration) has to cut benefits until payments balance with income (from the FICA tax).