Ah, the age-old question.
Acheron did a pretty good explanation.
As someone who acquires them myself then selling them.
I can tell you this. Cashing out Credit Cards isnt like it used to be in 2013.
Its an artform of its own, a craftsmanship if you may.
Its not an easy procedure by any means. Its very difficult, requires a lot of effort, research and investment.
These illicit jobs require a lot of skills and a lot of time forming those said skills.
Its like any other business in this world. (Cause pretty much this is what 'business' is)
Someone makes, someone buys, someone sells.