Because you're evaluating the effectiveness of one control in isolation, whereas the pension fund has a lot of controls, including e.g. an operational team which knows that wiring money to the Caymans is intrinsically high risk, a written procedure that they'll follow for high risk transfers specifically to papertrail up evidence in the event it is contested, a legal environment which will put the burden of proof on them rather than you if they did something that self-evidently stupid, the medallion guarantee program and associated regulation, etc etc.

Fraud happens. Financial institutions spend a lot of money defanging it; they also, when push comes to shove, have budgets for it.