Because in practice the places where it’s easy to go from BTC to fiat, and the places where you can easily money launderers, are distinct.
Financial regulation has caught up to most of the exchanges and it’s not always easy to get cash out of a system (and that’s not even mentioning loads of shadier exchanges near-guaranteed insolvency)
At the risk of splitting hairs: even Monero leaves a globally readable audit trail. It's just designed to be much harder / impossible to track. But no one needs even a warrant to start analyzing.
Random criminals are well advised not to trust their own judgement about crypto claims, nor their own prowess in operations security.
Keep in mind that eg lots of bitcoin exchanges have been hacked without any flaw in bitcoin itself. It's probably even easier to accidentally leak information, even if the underlying coin you are using doesn't.
