Because people are easily manipulated.
I think it better to say that people have always considered police errors as part of human nature. They have a tough job, and people are easily swayed by the view that policing will never be error free.
That said, I think it undeniable that the above view has resulted in insufficient oversight. Some years ago, there was a rash of deaths due to high speed chases on freeways. There was a blowback in many communities questioning the police’s reflex to engage in a high speed chase when the perp they’re chasing is just a garden variety crook; it makes no sense to put the public’s well being at risk to chase a guy at 100mph that just ran a red light or got $200 at a convenience store holdup, ESPECIALLY if you’ve already got the guy’s licence plate, make and model of car.
As a result, communities changed their policies governing when and where police are allowed to engage in such chasing activities.
I think the next area that could take review is the use of firearms on perps who do not have one and are not threatening. AGAIN, if a guy is a petty crook and decides to run away from a cop, the use of deadly force to stop him makes no sense.
The other area is force during apprehension. If a perp is willing to put his hands in the air, get into the back of the squad car all by himself and close the door, do they really need to slam his face into the concrete and kneel on his back to cuff him first?