> suitcase full of cash. From a security perspective, it has the interesting property that transactions involving one don't have to be publicly recorded and processed on a public, global, openly accessible peer-to-peer network in order to complete.

Indeed, but it also has drawbacks. You need to find someone with capability to use that much cash without suspicion.

Also, I think most of the money laundering is made across borders, from people willing to enjoy their wealth in a country different than the one they earned it. Think money earned to US to be used in Mexico, or nationals of a heavy currency export regulated country (rmb, ruble). Customs are very efficient at looking for cash moving through these borders.