Why would you say they are difficult/impossible to trace? It's publicly visible where it goes and where it gets eventually spent. Ill gained bitcoin even gets flagged and its very hard to spend.
1) You can track the transactions publicly, but once the crypto hits the wallet of someone that can trade cash for it, you've lost track of the criminal.
2) Privacy focused currencies like Monero make it exceedingly difficult to attribute transactions to specific individuals.
