They're wannabe military tough-guys with laughably worse training and culture, much of which tells them they should be pee-pants scared all the time (seriously—folks very-successfully sell some wacko training courses to cops; or, just look at the disconnected-from-reality-and-common-sense mythology they've built up, within their ranks, around fentanyl). That's the best-case explanation for that behavior. Sometimes they're literal machismo-fueled gangsters (yes, literally, as in, "involved in organized crime") and will punch down all day long to make sure they're feared.
[EDIT] Actually, school districts suffer from some similar problems, as far as being sold all kinds of ineffective and wrong-headed wacko programs and training, and too-frequently ending up corrupt little petty empires (OMG, the amount of money superintendents siphon off to pals...)—I think in both cases we may have opted for a level of devolution that's tipped at least a bit too far toward local autonomy, and could perhaps benefit from something like a professional management/officer corp that gets re-assigned every few years, like officers in the military, to prevent empire-building and make corruption and deviation into wacko-land much less likely. Though, I think that kind of thing's on exactly nobody's radar, so don't expect it to ever happen.
[EDIT EDIT] I mean obviously the problem's more severe and grave with cops, it just occurred to me that a lot of the problems in both spheres might be for organizationally-similar reasons.