Because if they are backed to only say 20% where did the rest of the money go? Did $80B disappear or what? And what are the reason to believe they did other tha

Because if they are backed to only say 20% where did the rest of the money go? Did $80B disappear or what? And what are the reason to believe they did other than conspiracy theories? Why would they not just buy $80B worth of bonds to collect free interest on them?