Because the social security surplus was used to fund the treasury instead of being put towards social security. The 1990s budget surplus that became the Bush tax cuts? Yeah that was the money that was supposed to pay for social security.

Surprising. Is that something common in Econ literature or something? It's definitely not the norm in poli sci journals, unless something's changed in the decade or so since I looked at many of those, since it just crowds out and distorts things that do contribute to the deficit.

It does hold government debt, but that's not really useful and puts it in a totally different category from "spending that contributes to the budget deficit".