It is far from trivial. What are you going to do if the money goes to an enemy country?

And while cryptocurrency are certainly popular with criminals, it is far from the only option for hiding transactions. As for the technology, if it exists, it is not very effective. The shadow economy is going strong even among average citizens, from drug trade to babysitting.

If governments can't stop even the most trivial kind of unreported work in their own country, how to you expect them to stop well organized international gangs, sometimes backed by nation states.