Debts are often sold to other companies to collect. Even after declaring bankruptcy, those bonds still exist, and third party companies will continue to hound you. Again, this isn't everyone, but it's still millions of people.
The problem is, there's no agency or administration to handle things like that, so there's nobody to stop them. It's the same people that collect debts on deceased. Sometimes they even collect debt that has been 'closed' or worse, paid! Yep. You paid off your debt, but it got sold to someone else somewhere in there and poor paperwork allows them to legally collect and garnish wages on something you paid.
Or worse, they just have the wrong info, and can legally collect on you, even if you didn't have the debt.