There was a terrible spate of kidnappings in the first half of the twentieth century, the Lindbergh baby being the most notorious, but the police have gotten good at foiling them. They mark the currency, alert all the banks etc.

Most businesses turn their cash over to a bank at the end of the day, so any ransom cash that the kidnappers spend or deposit in a bank is tracked very quickly.

Of course crypto can be tracked as well, but financial institutions are required to follow authority instructions and freeze and reverse assets that may belong to kidnappers. In most crypto there is no authority to reverse the assets.

You can have a system where ids of all wallets containing stolen crypto are kept in a database and nobody does accepts any payments from these addresses. However many crypto advocates are strongly opposed to such a system.

So in conclusion, kidnapping of law abiding citizens for ordinary money has not been happening in the us for a while because the fbi had made it clear that it does not pay. But crypto is a whole other business.

It is hopeful that for all current cases the perpetrators seem to have been caught. But i am afraid there may be more attempts before criminals are convinced it does not pay.