The constant change, reorganisation, restructuring, downsizing, outsourcing, contracterisation etc mean that there is no continuity in treatment of staff and thus the bully is able to hide the fact that he or she has a history of conflict with employees. There are only a handful of people capable of providing training in bullying investigation. Where HR want to investigate they are sometimes overruled. HR people get bullied too. Well, it's always worked in the past.
Narcissism is also common.
Targets are people who can see through the arrogance to perceive the empty shell behind it - and bullies can sense who can see through them. Sensing their survival is at stake, the bullying is designed to eliminate this perceived threat. People who are unwitting bullies can be helped by removal of pressure - then their behaviour improves. Sociopathic serial bullies probably cannot be helped at the moment. Work on psychopaths shows that the condition does not respond to treatment - in fact, treatment may make the condition worse .