Yeah, there's quite a bit of examples of necromancy that don't involve either of those artifacts, and some examples that cannot be tied to Azerothian Scourge at all. It's just in the cases of Arthas and Bolvar it's kinda weird. Like i said, unlike Kel'thuzad neither one of those had any arcane knowledge, let alone necromantic. So it had to be done through a medium. The only question - which one.