There abilities far exceed anything those classes are capable of thus again they are their own entities. What class is a titan? Void lord? Wild God? Naaru? No one is going to take you seriously if you start slapping titans like Sargeras with a class title.
Yeah I suppose the player characters are not the same. They tell Death Knights to suffer well as with creation they were instilled with an insatiable hunger to cause pain and suffering known as the eternal hunger they will feel immense pain and so forth until it is satiated. Something put in so they wouldn’t become too peaceful and relaxed. So then the solution is simply to scorch the earth so to speak wait them out and kill everything around them until they are hunched over in horrific unending pain.
Its an ongoing them Warlocks and Shadow Priests will eventually do our selves in the Death Knight is no different though instead of insanity its crippling pain and it’ll come much faster if not catered too.