Prosecutors require police to bring them people to prosecute. If a prosecutor starts going after cops, they've now pissed off a group of people they rely on the get their job done, so most prosecutors won't prosecute cops.
I imagine this could be fixed by states having a special prosecutor specifically for those that are now considered protected by qualified immunity, which is more than just police. Also we should get rid of qualified immunity, which is an insult to the constitution.
It's all connected. The police unions push back hard against reforms in the prosecutors' office that might subject them to more scrutiny.
https://www.nytimes.com/2020/06/06/us/police-unions-minneapo...
> In St. Louis, when Kim Gardner was elected the top prosecutor four years ago, she set out to rein in the city’s high rate of police violence. But after she proposed a unit within the prosecutor’s office that would independently investigate misconduct, she ran into the powerful local police union.
> The union pressured lawmakers to set aside the proposal, which many supported but then never brought to a vote. Around the same time, a lawyer for the union waged a legal fight to limit the ability of the prosecutor’s office to investigate police misconduct. The following year, a leader of the union said Ms. Gardner should be removed “by force or by choice.”