That's not how LE function. Think about them like this, during investigations they are on the clock as civil servants. They have oversight, Imagine attempting to explain spending taxpayer dollars on telegram chats. Not happening.
This is what they do, they place orders anonymously, document what they receive and coordinate with postal inspectors to zero in on the package origins. From there its pretty much game over. However they drag their feet and it takes years to get an indictment. All a source has to do is print money for a year and rebrand. It's fairly straight forward.