It's because you only hear about the bad things. Sure there are some crazy cops like everywhere else. But all in all I'm certain most cops are good people who just want to help citizens and remove the bad guys.
Defenders of police misconduct tend to cite the "it's just a few bad apples" line without noting the full adage is "a few bad apples spoils the barrel".
These guys are supposed to uphold the law, instead they prop up murderers and accuse the justice department of railroading “innocent” cops. See “Police union blasts ‘sham trial and shameful’ Van Dyke guilty verdict” [0]
0: https://chicago.suntimes.com/2018/10/5/18460941/police-union...
Contrast this with the principles of policing by consent [0] that define the ideals (though not necessarily the reality) of policing in the UK and many other countries.
The thing is is survivorship bias often shows in the media. most police encounters are not reported in the media because it's common and everyday. But the very few that turn into a big production get reported because it gets ratings. So through survivorship bias, confirmation bias, and availability bias, the problem seems bigger than it actually is.