But, you can't buy a new car or house with that money. In the US we have a law that forces banks to disclose to the IRS whenever we deposit $600 of cash or more into our accounts, so there's really nothing you can effectively hide from them except for some pocket money.
While honest people are hoping to pay less taxes, criminals want to pay taxes so they can use their money "legally".
Answering to both of you, because the answer has common parts.
In my country, every bank transaction that exceeds 500 euros triggers a flag, which is just a notification to the authorities. There are some exceptions, for specific transactions that are mostly salary payments.
I have watched a great video in the past, which I can't find now (sorry for that), which showed that at some point, the criminals drop some of their money in the banking system, trying to launder their money. Strange, isn't it? They intentionally mix their money with other sources, in an effort to hide their traces. So, the criminals don't always try to use FIAT, P2P, but they also use the banks for money laundering.