It is probably because there is a difference between getting mugged and being scammed. With falling victim to SBF's scheme, you probably believe you are partly to blame and you had to face that fact yourself, did some work on it mentally at the very least in the background. Him getting punished does not make a difference, you probably punished yourself and that is what mattered. With a violent mugging encounter, you are blameless, so you need to see the other party get punished to process and to feel that the world will get slightly better / safer with the delivery of the punishment.