I like to think of it prisoner's dilemma style: the optimal amount of fraud is zero, the same way the optimal outcome for both prisoners is to both cooperate. But the equilibrium at 2-cooperate is not stable, somebody will gain more utility for themselves by defecting, the same way with 0 KYC on any and all transactions fraud is laughably easy.

Thus, the global optima is one where there is no fraud and everybody cooperates, but it's not a stable optima and we slide to the real world where there are tradeoffs for preventing fraud, and there reaches a point where rational actors deem the tradeoff unacceptable and thus accept some level of fraud.