Because, unless you go completely meta on them, they are the paragons of virtue, who can do no evil, are completely pure and spotless, and I [censored] HATE TRAVEN! With the Morrowind chapter, at least they acted like a guild! I haven't played the DF one, nor looked at the quests, so I cannot give a fair assessment.
Also, I find the charge against necromancy to complete and utter propaganda, perpetuated by cultists of Arkay, which I suspect Traven to be a high ranking member of.
What does it mean "to go meta"? Even when I googled that phrase, I was like "huh?"
Well didn't both OB & MW act like guilds, though? I see your argument that the TESIV version was more black & white and they were all good do-ers, which irks me as well. Did you simply just like the quests in TESIII better or was it the way the NPCs talked or something?
To me, Necromancy is not evil at all; just its connotations. If people were to practice it more and understand it better, they would find that in theory, it could be used for so many useful purposes, like bringing back dead plants and trees and crops, in particular. You'd probably have to do a little mix n match with various schools like Alteration, Restoration and Mysticism. But primarily, bringing dead life back to life could be a great and useful practice, if only citizens of Tamriel weren't so threatened by it.