The US Constitution requires them to exist. Congress keeps passing laws screwing with their operation. Such as requiring pre-paid funding pensions/healthcare for people who have not been born yet.

> they can't declare bankruptcy

According to Amendment 14, section 4:

> The validity of the public debt of the United States, authorized by law, including debts incurred for payment of pensions and bounties for services in suppressing insurrection or rebellion, shall not be questioned.

https://en.wikipedia.org/wiki/Fourteenth_Amendment_to_the_Un...

According to that clause, especially the bit with the comma, Congress & courts have held that government bodies cannot discharge debts in bankruptcy. They go through Chapter 9 of the bankruptcy code. Every other chapter in BK allows for some debts to be wiped away.