The people who would be responsible for prosecuting the officers are the same people who prosecute everyone else in that jurisdiction. Through all those other p

The people who would be responsible for prosecuting the officers are the same people who prosecute everyone else in that jurisdiction. Through all those other prosecutions, they tend to develop a strong working relationship with the police force and are typically on the "same side." It's also very rare for police to testify against their colleagues — as you might imagine. That's not the only reason, nor is it always the case, but it's one of the big ones.