This could be a stretch!
It may not be.
Arthas is depicted all realistic here, and yet the Scourge are still like the models ingame. Is there an obvious answer here? Yea, it's that they didn't want to show an actual decapitated head.
But! Perhaps it's because of the nature of the Scourge. Take a look at these guys.
Concept Art for Cataclysm, never made it ingame.
Zuraal here is obviously a lot more Void-Like, but you can see how the Broken were changed by the Void too. Shapewise, we see some similarities.
Could this be because of the nature of reanimation through Dark Magic such as the Void? Which we know is tied to the Scourge already.
Are the souls being ripped back through the Void in an uncaring way, thus ripping apart their form, and re-emerging into reality as grotesque husks?
Thoughts?