Because our criminal justice system isn't (and shouldn't be) based on the principle of doing to the criminal what the criminal did to his or her victim. Punishment in a civilised society isn't revenge or 'an eye for an eye'. Many would argue that punishment necessarily involves an element of retribution (as well as deterring others and rehabilitation or reforming the criminal), and it should certainly reflect the outrage which society feels at the injury it receives when a crime is committed. But that doesn't mean we should lower ourselves to criminals' level and treat them like they treat their victims.