Due to the legal concept of qualified immunity, it is very difficult to sue the police for civil or criminal liability. I understand the intent of qualified immunity. The idea is that a police officer did the best they could knowing what they know, and it is very easy to look at things after the fact (hindsight is 20/20), so it makes it easier for a police officer to do their job without constant worry of lawsuit. However, it has been abused to the point that it is next to impossible to sue for criminal/civil liability.
Additionally, there is wide conception that police do not police their own (look up "blue wall of silence"), and police unions are very strong, so it is extremely difficult for an outside organization to police them.
Case in point, it is argued that if it weren't for the cell phone video of George Floyd, the police officers would not have even been charged with misconduct and would still have their jobs, much less criminal charges.
If you have an organization that has a monopoly on violence and there is not a good way to keep that in check, THIS WILL LEAD TO MASSIVE PROBLEMS.