It sounds great to say, why not just go after the criminals? The practical reason why not is that people who launder money create an incentive for crime. Every thief knows how much they are likely to get from a fence. As an ethical matter, why shouldn't we punish people who knowingly profit from people committing crimes-- often horrible crimes?
I really recommend reading "The Billion Dollar Whale" because it is a good book and shocking. It is about an individual who really just stole billions of dollars from the Malayan government and then tries to hide it. Every year his fund has a reputable auditor, but every year it is a new auditor, for example.
Goldman Sachs facilitated the crime by making huge transfers with very little diligence. They asked for huge fees, not just relative to the amount, but on a percentage basis. Probably because they knew something was shady and they figured they would get sued. Also, they were aiding a crime but as you say, it all will be just a fine no charges.
GS admitted guilt and is paying almost a $3b fine for this. And IMO they should.