You're wrong! There's a spell called "Raise Dead," which is essentially the same thing.

Not quite. The "Raise Dead" spell is exactly that – a spell that uses a hero's mana, and its effect depends on the level of skill in a particular magic. And, by the way, it's not only available to Necromancers. The new racial ability, however, now charges during battle, as your own and enemy units die, and the effectiveness of the used ability depends on how full the scale is (and the level of the corresponding skill, presumably). These manipulations are only available to the Undead. The end result is the same, but it's achieved in completely different ways.