The best answer I can give you is that by our continued reporting of such fraud, we help build aggregated cases for law enforcement to investigate these criminals. Why such a vague answer? One reason is that the criminals in question are almost never located in the U.S. If each reported case of fraud is treated as a singular case, then crossing international lines of jurisdiction to apprehend a criminal who stole only $5000 appears very daunting to a local officer of the law. Reporting to http://ic3.gov draws the concentrated attention of U.S. law enforcement.
See this very enlightening article on a village in Romania in which these scammers thrive.
More indictments on 01March2011 Read about it here.