Because "police beat up innocent people" is a non-argument. You could replace "police" in that statement with just about any group of people and it would still hold true.
Sure, but when anyone not from this specific group beats up someone, the criminal justice system generally prosecutes them. And if the criminal justice system fails to do that, we all still generally agree that the attacker was in the wrong.
Meanwhile, when a police officer beats up or even murders someone, often an entire city has to protest for that violent criminal to even be charged. And their coworkers, purported employees of the public trust, purported believers in law and order, go on and on justifying the assailant's actions as if they should be normalized.
