Mostly, it's about the value or lack thereof of utility abilities.

As a thief, I've killed PCs (effectively, regardless of what actually did the killing damage) by:

- Using hide/sneak to catch them almost dead from another fight and finishing them off
- Lagging them out entirely or almost entirely with trip and cheap shot
- Knocking them out at just the right time for crucial spells to wear out
- Stealing their food and potions, later leaving them starving and out of movement

to give just a few examples out of the wide variety of things you can do with a thief.

None of those things is particularly useful against NPCs. Tiamat doesn't freak out and make a critical mistake because she went for her teleport potion and you'd already stolen it.