Possibly because it's easy to say that afterwards, but it isn't directly relevant to the bystander effect. Sure, it's the sensible option, but that isn't on many people's minds at the time because that isn't how biology works.

Also, although it may be beside the point, you could argue that giving them what they want will just increase muggings. The way to sort out the problem is to relax laws around people helping out victims in cases like this.