On the ground, a large portion of (consumer) debt collection is based on pulling fast ones. The people in debt are not aware of their rights and protects, and the debt collectors deceive, sometimes stretching the truth and sometimes outright lying. Threatening with prison sentences and even posing as cops are mentioned in the linked article, even though one is a fiction and the other an outright felony.

So really, your question "why should a debtor pay...?" is always answered by "the collector convinces them to." Giving the consumer rights and protections is, at this point, insufficient. Their current rights and protections are not being exercised, due to ignorance. That's something that needs to be fixed.