Why is that? I mean they just broke the Lich King's mental control. The necromancy that animates them did not suddenly alter.
The Forsaken are zombies that make an effort to look less disgusting. Thus they stitch back broken flesh, they replace broken jaws or cover missing eyes with leather, they use preservatives to reduce the rate of decay. They are still zombies. They walk in a stooped posture. Some
cannot even speak Common anymore .
They are also inherently magical beings animated by malign magic that was known to take many forms. The Scourge was NOT just zombies after all, there were other forms of undead among them and each of them was far more dangerous than the shambling hordes. A creature that looks like a zombie but acts with intelligence would be a greater threat.
You say the just look dead. But that cannot be right. They are not intact walking corpses after all, not pale humans that smell a bit funky. Most of them are missing the flesh from their joints, missing eyes, jaws. It's not like they are freshly animated; they were running around as packs of shambling zombies for a couple of years before they started breaking the Lich King's control and that understandably caused a lot of wear and tear.
This is the one thing I would most like to know. In general Blizzard has not really told us WHAT happens when people die in WoW. If the act of necromancy forever stains the soul and damns it, that is truly maddening. This reminds me of Crusader Bridenbrad. The best the Naaru could offer him was to save his soul. Which suggest that his soul WAS damned.