The easiest way to move money from one place to another, is to move it using the banking channels.
With an account anywhere in the world, money can be moved to any account across the world by doing a transfer.
For example, let us examine the working of a drug cartel.
The drugs are sourced from someone and he has to be paid, probably in cash. If, for example, the drugs are originating from Afghanistan and are being purchased by a syndicate in the US, they have to make payment in cash in Afghanistan while they have to collect the payment from their distributors spread over the entire country.
The distributors in turn have to collect payment from those who sell the stuff on their behalf. Most of the sale happens in cash.
Thus, the cash has to be collected and sent to the organization.
For this purpose, the agents who sell the drugs set up a business account which is supposedly doing a legitimate business, like a restaurant or a coffee shop. Only thing is that these businesses do not really do the amount of sale that gets deposited in their accounts. So, a restaurant may be just a dummy front which deposits 10,000 USD every day into the account and then makes payment to others from these deposits. That is how cash gets introduced into the legitimate system. Once the cash has entered the banking system, it gets transferred to others, supposedly vendors, who too are fake front accounts.
A gang of distributors used Spotify stream to recently collect money from users and direct it to their bank accounts. Goes without saying that those users were all fake and all payments were received in cash or small payments that go under the radar of any money laundering detection system.
Banks are now using smart software to flag such operations. They detect unusual cash deposits and flag them to the authorities.
I am sure the crooks are already two step ahead of the detecting authorities.
In a way, cash transactions can help detect money laundering better than bank transactions. Thus, for the authorities cash transactions are a necessary evil.