In any other situation, testimony or evidence from such an easily impeachable witness would get laughed out of court. Why do we allow it from cops?
If a journalist or academic gets found to have knowingly fabricated even the smallest detail it immediately calls into question their entire body of work, and even colleagues they may have worked with.
These are federal agents and the best standard we can hold them to is "oh, we don't have direct evidence of other crimes so we'll just assume the first time we caught you was the first time you did it"
They sold him details of the case for bitcoin into their personal wallets. It wasn't part of a sting or intel operation. The same cops that built the case against him were themselves indicted for money laundering payments they received from Ross. Multiple independent agencies and field offices with rogue agents undermining the cases they were building in real time for personal gain that was contemporaneously worth less than a million dollars.