These are debts sold at >10% of face, often smallish debts that have accrued massive interest. Some agencies specialise in persistent calling. Some specialise in deal-making. Some specialise in legal proceedings. These are the ones we are talking about.
To work, they need to keep legal costs low. They often don't bother to have a case, because they're happy to win just the no shows. That's the formula. Buy 100 notes. Take them to court. 80 or so will no show. You can then try to seize the their assets. The whole thing is premised on the fact that these defendants don't open their mail. Either side making a case to a judge is rare.
It's a misuse of the legal system, and the court shouldn't allow it.