I skimmed a lot of comments and didn't see one directly addressing the question posed in the summary.
Basically, 35% of Americans have debts in collection status because it's easy to have an account go to collections and then linger there forever. You can imagine people's "debt responsiveness" as being exponentially bound to the time since the last payment. A debt that recently had a payment made is almost certain to have its next payment made. A debt that's a few months late has a decent chance of getting a payment made soon. A debt that is 6 months (or more) late has a very low chance of ever being paid again. This is why debt collectors buy/pursue old debts. The original creditor will likely accept pennies on the dollar just to get something out of it, while the collector wants to obtain the whole amount. If they can even get half, they come out way ahead. It's a profitable business.
I used to work in this industry (wrote software) so I could tell you some things about it.