The reason for this is that restitution, or any criminal punishment, is not solely about making the victim whole in a monetary sense. There are at least three other reasons for requiring restitution to be made even in the case of a wealthy victim.
First, part of the point of the criminal justice system is to make victims feel as if society protects them. They need to feel that people who commit offenses against them will be punished. If restitution were not made, the rich person might feel as if that meant it was “open season” on stealing from rich people. This would reduce their trust in our society.
Second, and very relatedly, the criminal justice system needs to act in such a way that it reassures society as a whole that it is protected. Criminal punishments are partly about sending the message that society will enforce its rules. This helps to keep the society cohesive.
Finally, the criminal justice system is partly about punishing offenders for their actions, not the consequences of those actions. Someone who steals from a wealthy person has committed an act that is a violation of the other person’s rights. Therefore, the person deserves to be punished, regardless of whether the victim was harmed in a tangible, monetary way.