Why? We do not have punishment proportionality in many aspects of life. Kids are spanked for simple disobedience. People are fired for stealing meager sums. You can be killed for jaywalking. It makes more sense for sentences to be proportional to the desired effect.
OK,an extreme example: there was a day not *that* long ago when you could get the chair (in some states) for rape.In those states where it was once true they certainly had the DP for murder.In such a situation (DP for rape *and* murder) a rapist has a strong incentive to kill his rape victim to keep her quiet,whereas if rape gets 20 years and murder gets death he has an incentive *not* to kill her.