Even if some millionaire shouts that, thief would have to rob a bank to get to your money or kidnap your family member and ask for money and neither of those things are very easy to do. In the case of crypto millionaire, its enough to beat him up until he gives you seed which is much easier.
Most fiat ransom in my country are done in a way that no trace is left. The crypto ones that we read recently, the attackers are known and arrested. Cash is the most anonymous way to collect money from someone, unlike cryptocurrencies that can be traced. Some people kidnapped the relatives and demanded cryptocurrency in a way that the victim is with them also just like it is in fiat ransom. They will not release the victim until the family pay the ransom or police was able to trace the kidnapper. Most kidnappers are nit traceable.