Which coincidentally you need to do to cash out or cash in.

Yes, and no. You do need to cash in at some point, but you don't have to do it thru a public exchange. People do sell physical wallets for hard cash. And even if you do use an exchange, when I last looked into crypto the common currency for drugs (monero) was obtainable on exchanges that didn't have KYC rules. Outside of exchanges, you can also transmit currency directly to other parties, and once you use tumblers and other anonymous platforms, tracking becomes extremely difficult. It's not impossible, but it becomes troublesome enough that unless you're a big fish/crime lord/whatever, the FBI/interpol/whoever isn't going to be bothered wasting resources.