They don't try very hard. If they do, they get demoted, discharged or dead very quickly.

Efforts to change the system from within have been going on for over a century now. We've seen the same patterns of corruption and brutality surface, get confronted and then dismissed with the same inaction.

No, they're all bastards to the last, and even those who quit have to confront the injustice in which they've personally participated just to sleep at night. Law enforcement in the US exhibits a lot of similarities to Heydrich's Sicherheitsdienst and engages in the same kind of overpolicing in order to justify its violence and continued existence.

As for those unwilling to hear it, or who buy into the pervasive pro-police propaganda in the media, they put themselves at risk, for the face-eating leopards are very much on the prowl.