Necromancy is about reanimating dead bodies, especially when it comes to preparing bodies, like adding spikes or wrapping zombies as mummies, strengthening the joints of skeletons, etc. So how is it too broad of a school of magic?
One doesn't shoot black magic out of their finger tips or something silly of that kind of sort. It's, pretty much, just reanimating dead bodies. Ripping someone's soul out would be mysticism, and summoning a ghost would require one to be skilled in conjurations, as that person is ripping a soul out of the Dreamsleeve and bringing it to Mundus.