What do you imagine they should do with it? Open a savings account at the bank? Store cash in a big vault somewhere?
They had to do something with the money and they did what anyone would consider the safest investment. They bought government bonds. That accomplishes the same thing as when you buy a government bond. You give your cash to the treasury for a promise to pay it back with interest in the future.
Whatever you think of SS, and I'm against it, that's not stealing it.
They didn’t just buy bonds...some went to highway projects and social welfare programs. The money did not stay in the fund, if it did, it would be there. THEY STOLE IT.