Their job is not to solve crimes, their job is to get people convicted, the subtle difference being that they’ll turn non-crimes into crimes (for example, they’ll chose to legally interpret things which can go both ways as crimes which require prosecution, which is why one often sees kids criminalized for childish bullshit) and it doesn’t matter if the person convicted is innocent, all that matters is that somebody got convicted (so, for example, they won’t try and find exonerating evidence).

This partly explains their tendency to take an adversarial posture towards people who aren’t from their group, also partly explained because that posture itself indirectly feeds back on them (people are weary of them because of how act towards the general public, which in turn makes them feel apart and suspicious hence they behave even more so) and partly because they do tend to get exposed far more than most people to the seedy side of humanity all with a judgemental mindset and an aim to see crimes, so even a lot of the stuff they see which most people think is just silly fun (say, most drunkenness), they’ll see as crimes.