Cops are not lawyers, let alone judges. I see why this is necessary and often even a good thing, but it does ensure that any police training will always be based on heuristics that a non-lawyer of average intelligence can apply in-the-moment. Choosing good heuristics is hard. Just like building effective institutions. So of course, society has abdicated most of its responsibility to even try.
@M_Dub Yes, one of the many bad solutions in current use.
And the people that are supposed to reign in bad behavior by cops (DAs who should want to avoid losing cases, judges who approve warrants and preside over trials, public defenders, oversight committees, internal affairs) either have every incentive not to or have no resources to do so adequately (because politicians have no incentive to provide those resources).