Also anyone that tries to fix the system from the inside tends to get targetted, at least by HR if not by their peers whose corruption they are threatening. The bad apples are in control and have a system to either turn new apples bad or toss them out.
And the police unions are used to avoid reform from above, since pretty much any attempt to discipline is challenged with whatever means available.
And they are a gang with state resources available to help enforce their will, plus inside information about how investigations are run and access to those doing the investigations, should their other lines of defense fail.