Because it represents a very large opportunity for the advancement of income for attorneys AND because the Big Record Labels want them to.
The RIAA doesn't sell anything. They are a marketing organization of lawyers. The Big Record Labels are the ones who have product to sell.
The RIAA is the attack dog that the Big Record Labels use to do their bidding.
The genie is out of the bottle and he aint going back. Just wait till P2P networks start doing transfers ala SSH type comm. Or Freenet really takes off. There's other apps being worked on that you, the user never really have the "whole" file for whatever your going after on your 'puter; it's ALL distributed. How is the RIAA going to prosecute that?
Do the record lables have a gripe? Sure they do. However, If they got on the bandwagon and started selling per song online, they would make a mint.
Just my $0.02