I mean nobody is expected to investigate their own group. That is why things like judges/courts, prosecutors, civil/town/city observation groups and federal agencies are there.

The problem arises when you have cases like: https://www.msn.com/en-us/news/crime/lawsuit-claims-loveland... (series of false DUI arrest to bring in cash) and the whole police department is not raided on the spot.

Courts just drop cases because: a) qualified immunity and b) an "allergy" to having the D.A. and other agencies step in and do the investigation.

P.S. Hold your D.A. accountable in your area when you hear things like that. It is more important in the long run to have a few less street criminals behind bars and more corrupt "apples" in.

Edit: typo