If/when the movement of cash/crypto is peer-to-peer or interacting with a machine only (a machine that gives quarters for dollar bills or a DEX, for example) KYC is entirely unnecessary.
Dealing with large volumes of cash is very difficult.
It's physically large so you can't move it between countries with drawing attention from customs and security. You can't exchange it between currencies easily since brokers are required to implement KYC/AML. Banks have automatic triggers on their internal systems to notify regulators if you deposit/withdraw large sums.
And if it's being tracked then washing it requires you having to go through exotic means like poker machines since all simpler options have been locked down over the decades.
It's one of the reasons crypto is so popular with states like North Korea, Iran etc because it's scalable enough to allow them to move billions.
