We shouldn't mix up "chance of going to jail" with "severity of punishment". Potential criminals easily dismiss severe punishment, but they don't dismiss near-certain punishment. Quick and reliable punishment is highly effective. Increasing the severity is weakly effective.
There is also the simple fact that people in custody are physically separated from potential victims. This very obviously works.
There is also the simple fact that people in custody are physically separated from potential victims. This very obviously works.