Why do credit Card companies wast time and money perusing worthless debt?

I was watching a news special the other day and it said that credit card companies and collection agencies spend lots of time and money trying to collect worthless debt ? They pray on poor uneducated people to try and collect debt that is not even worth pursuing. When i loan money to a friend and they don't pay me back in like 2 or 3 months i just write the debt off as a loss and don't loan any more money to that person and that's it life goes on. I have heard of them pursing someone even 15 years or more . that is crazy . I want to know what is the thought process behind it ?