1. Because no police leads to an intolerable state of lawlessness, and the people will beg for an overcorrection.

2.. The people keep themselves in check either by doing violence themselves (state of nature) or outsourcing the violence to an accountable third party (police). Doing it yourself works for a small minority of those cut out to be mafia bosses and warlords, but most will choose to outsource. There is no utopia without the credible threat of violence as deterrence. If you don't punish violent criminals severely you don't have a functioning society.