Believe it or not, authorities do occasionally catch cybercriminals. Just last month, who may have been involved with the ZeuS/Zbot Trojan.
Not that it did much good. , security researcher Troy Gill declared a new variant of Zbot "the most predominant virus/phishing campaign right now."
And that's a big part of the problem. As with drugs and prostitution, if there's money to be made in an illegal activity, people will be there to make it. Arrest two, and 20 more are waiting to take their place.
If you have no scruples, cybercrime makes a tempting way to make a living. It can be highly profitable, you can do it from any place in the world where you can get an Internet connection, the punishments are lax compared to violent crime, and your chances of getting caught are relatively low.
Which brings us to Victor's original question: Why isn't someone arresting these people?