Punishment is not effective for most people in reducing crime. One thing that is effective is increasing the perceived risk of getting caught. People rarely commit crimes when they are sure they will be caught.
There are plenty of medical interventions, particularly for mental illnesses, that would dream of 1/3 effectiveness. Now consider the fact that recidivism rates are often much lower than 2/3, and in many cases are closer to 1/3. That tells me that punishment actually is quite effective at reducing crime.