Cops don’t prevent crime, not in any significant way. Their job is to enforce the law, typically by responding after a crime has already occurred. Because they are trained to primarily respond with force and to believe any and every thing in existence absolutely will try to kill them at any moment, and to treat everyone the encounter as guilty and sources for incriminating information, they act as hammers treating everyone as a nail. They are dangerous, paranoid thugs never to be trusted.