Isn't this honestly largely due to the police union rules and contacts, which (unions) are a key part of leftist labor relations?
Does that answer your question?
Companies can fairly easily break a union. After call, why does Ford need a Detroit plant when they can use Mexico.
But governments are lead by politicians who get money from unions. Union supports leader, leader pays union, union supports leader. No incentive to keep costs down at all. It's actually better for politicians to have strong public unions.
Teachers unions make it pretty much impossible to fire someone. You pretty much need to molest a child in places with a union (after you make tenure).
No police union contract includes a statement that the officers are immune from criminal investigation and prosecution, and most of the egregious events would clearly result in investigation and prosecution (if not necessarily conviction) if done by non-police. That these cases don't get that far shows the effects of a general cooperation between police and the judicial and political system---DAs don't prosecute and courts won't convict---all backed by a social system that encourages that behavior: you won't get elected on a police reform platform anywhere in the US.
Hence the US states' and cities' unfunded pension debt crisis. Only solution to it in democracies is for non government employees to actively participate in local elections.