Because the criminal justice system isnt solely about revenge.. It has to draw a fine line between many competing objectives for instance; deterrance/punishment/rehabilitation/public safety and so on..
Not to mention reasons like many ridiculous laws on the books, victimless crimes [for instance drug possession], circumstances [a drug addict stealing for their habit is interesting as some drugs like benzoes and alcohol withdrawel will kill you or otherwise a poor person stealing to feed/clothe family] having a system like america is counter productive with their 3 strike policy where it counts all crimes with no distintsion.
Id say that crimes should be on a case by case basis for instance if someone went out with the intent to kill someone lock them away for life however if someone was at a lub and a br fight broke out say he pushedsomeone nd that someone fell and cracked there head sending him to jail for years would help no one.. not to mention the problem that people with records find it very hard to get work and thus will almost certainly commit crime again.