I just want to jump in here and say that it's definitely not an issue of training: I was a cop for 4 years, and while I was shitty at most of it, I do remember having to take training on not shooting black people at least twice a quarter.
I think it's the issue that being in the police is taken to mean that you have the power to make people do what you want, so that attracts bullies, which pushes out people who aren't bullies, and then the cycle grows. I know I never saw anyone abusing their authority except once, and when I did I spoke up and shut them down, even though they were my supervisor.
One also has to remember that we have bias due to the way the media works - the problem isn't non-existent by any means, but you aren't going to hear about the "Good" cops because it doesn't make for a good story if they deescalate the situation and everyone leaves peacefully. You only have a story, something that will stick in your head, if something goes wrong. I forget the exact term for it, but it's an established psychological thing.
And of course it all folds back into the identity politics the U.S is drowning in. Most cops identify as right-wing, and therefore basically follow whatever the party line is. The left-wing cops tend to be similar, but have a better chance of not being radicals.
Also relevant is that the cops I met tended to be a bit dull. They weren't exactly the most rational people - a consequence of rational people not wanting to be cops because then they would be surrounded by irrational people.