The point of the punishment is to tell you how much we don’t want you to do it. You get life for murder because we really don’t want people killing people. Fleecing them out of money is bad, but we can (literally) print more of it. Once someone is dead we can’t bring them back.

Your analysis is flawed because you’re looking at the problem from the vantage of collective outcomes. A murderer is one person who is making a decision to take an action that can end one or many lives. You want them to weigh that heavily as an absolute cost (I will go to prison forever if caught) not some relative cost (well if I just murder a little then I’ll only get a little punishment).