I don't hate cops. I have a problem with "we've investigated ourselves and determined we've done nothing wrong."
Police are powerful institutions who are in many ways structurally immune to outside accountability from friendly DA oversight, powerful police unions, qualified immunity that shield many from consequences in instances of bad or questionable behavior. Armed state agents killing people under murky circumstances for instance is intrinsically a concern for citizens. Dash cams and body cams 'malfunctioning' no questions asked is unacceptable. Civil asset forfeiture. Corruption. The war on drugs and the associated violence and mass incarceration. The thin blue line. All reasons to question the role of police departments as an institution in our society. I don't have any answers or horse in the race in any particular way, those are just reasonable critques of the police of the top of my head. There are plenty of reasons to critique the police institution that make no moral judgment of rank and file officers as people.
Most people who've grown past their a blue haired college socialist rage against the system phase recognize that most cops are just guys doing a hard job to the best of their ability, but that doesn't give them a free pass on critiscm. When fucking up at work can mean innocent bodies on the ground or erosion of constitutional liberties, with limited recourse from the citizenry, well it's not exactly a trivial matter.
The reason nobody posts videos of people losing arguments to cops is because it doesn't matter. Cops breaking the law, crossing boundaries, lying, misusing force, and abuse of power in general is automatically of concern because they are the enforcement arm of the state, with implicit the authority to use violence to that end. By contrast, Joe shmoe's misunderstanding the 4th amendment in some misguided attempt to speak truth to power at some hypothetical traffic stop is of no consequence. Why? Because he has no power. He's just a schmuck who's wrong. So nobody cares.
At the end of the day, any serious critic of the police is concerned about accountability and abuse of power.