At a minimum, watch 100 videos. I did last night, only took about an hour, it's easy to find some to nitpick, some which are ambiguous ... and plenty that are totally horrifying.
If you can watch 100 videos in a row from Greg Doucette's list and say, "the militarization and use of force tactics of US law enforcement are not a problem" then I'd like to hear why you think so given this evidence.
Otherwise you're not speaking from an honest grappling with what these videos contain.
Does this mean that people just do not care, or there is only some minority who thinks that the police is too violent and the rest is ok with that?
Just as a synopsis, systemic racism became a more subtle segregation. POC (People or Person of Color) were systematically made to appear more violent and criminal-like over time. Combine that notion with an idealized notion of the police as hero figures and you have a recipe for rationalizing violence against POC.
Mobile phone videos allowed us to see from the victim's perspective just how brutal police have become.
"How it happens that all those mayors and sheriffs are still in the office if police brutality is such a big issue?"
To this point, you need to understand about voting districts and how POC voting power has been diluted and prevented over decades.