And often, they don't get removed because the "good" police protect them with a thin blue line of silence. Those "good" police aren't really good the very instant they protect the bad.
So the police develop a thick skin, and close ranks to protect one another. They couldn't exist otherwise. Its understandable. But it leads to problems when things like police hazing perps is tolerated. It leads to deaths in custody etc.
I blame the leadership, which tolerated hazing and mistreatment. That has to be squelched instantly, or it grows without bound until somebody is seriously injured or dies.
Policing in other western countries indicates otherwise. For example, if police were held personally liable, as a strict matter of law, for the cost of their mistakes (just as individuals in the private sector bear the cost of their mistakes), they would tend to act more responsibly. Britain is an example of this.