hot take: I suspect a similar effect goes on in staffing police departments. I still believe that a majority of police officers (especially the one that don't make the news or go viral) are good cops who have secure egos and really want to uphold the law, but a fraction of officers really are motivated by cracking skulls and ego tripping (i.e. those things actually feel good to them, and are a significant part of why they are taking the job). The reason why these behaviors haven't been ruthlessly stamped out is because if it were completely not tolerated, then departments would have significant shortages, so there's an implicit "perk" of being able to ego trip and push people around and exert dominance (of course, up to a limit) which is a necessary cost of hiring.