Why must necromancy be disgusting?

[Marillion]

Why must nearly every undead creature worth using as more than a mook absolutely disgusting?

Emphasis mine. An undead creature is not alive; the necromancer has given them enough life-energy to cruelly force the remains of the deceased creature (which may have been lying in the ground rotting away for years) into a mocking semblance of life, enough to make it capable of movement, but not enough to stave off or reverse the decomposition process. Why should he invest such power in the creature? It cannot feel, nor did he raise it for its delightful company - he raised it for one specific purpose, a purpose that does not require a pleasant countenance.

It isn't muscle, blood, and bone that's animating the creature; it's magic. And magic is a valuable resource to be invested wisely in one's creations.

EDIT:

I'm not talking about that, I'm talking about the freakish things with six arms three legs and no apparent head....

That sort of thing.

Simple shabby zombies isn't what I mean.

Many of my points still stand. The necromancer who created this thing didn't create it for its charming personality; he created it to kill things. If it doesn't need a head to do that, why give it one?