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 because that posture indirectly feeds back on them (people don’t treat them as they treat other people) and partly because they do tend to get exposed far more than most people to the seedy side of humanity.