The reason why punishment does not deter crime is because people who commit crimes usually do so because they are out of options, or were not given other options to begin with. So if you increase the severity of the punishment, you are merely making it more stressful for the people to commit the crimes, rather than deterring them.

That’s my take. And I don’t have a damn criminology degree to come up with that. (Not to say it’s necessarily true, but it rings true to me.)