One word: qualified immunity.

1. It's a law that protects people from civil suits.
2. Combined with unions, like the Fraternal Order of Police and the International Union of Police Association, and federal court rulings, like DeShaney vs Winnebago and Town of Castle Rock vs Gonzalez, which ruled that police agencies are not obligated to provide protection of citizens (AKA police are well within their rights to pick and choose when to intervene and to protect the lives and property of others - even when a threat is apparent)
3. Asset Forfeiture abuse funding said unions

All 3 create a sense of entitlement. Police view themselves as untouchable. That on-so-sacred "thin blue line" won't be broken even in circumstances that should have been reported well before it was necessary. And because of this, most police operate with justified impunity and without accountability.

If there is a National Bad Cop Registry, issues like this wouldn't exist.

"Internal investigation" has become just a buzzword. A buzzword that replaces the idiom 'fox guarding the henhouse'. How can you trust the outcome?