If it was as simple as necromancy, the Forsaken wouldn't have needed the Val'kyr in the first place. Necromancy damages the soul upon reanimation and the undead becomes slave to the person who raised it. It's unclear what method the Val'kyr use to resurrect people, but those resurrected have their souls intact. Sylvanas wasn't resurrected by Val'kyr. Her soul was reaped by Frostmourne and corrupted by Arthas.
EDIT: Also, Val'kyr have to sacrifice themselves to resurrect Sylvanas; to bring her back from that tormenty vortex shit her soul goes when she dies. For everyone else, Val'kyr just escort souls to their corpses like Spirit Healers, except into undeath instead of full life. And only humans.