Same reason why every bandit, wolf, bear, boar, cliff racer, wild guar, baliwog, and riekling wants you dead: because http://tvtropes.org/pmwiki/pmwiki.php/Main.EverythingTryingToKillYou%5b/url.
Exactly, but that's also the trope Hellmouth is railing against. It's particularly telling that the article starts with
"Video games struggling for creativity will invent unlikely obstacles", but it doesn't have to be that way.
Right now the only view of Necromancy presented in Oblivion is that of the Guild of Mages, which is almost pushing Necromancy as if it were the Red Scare. The Necromancers themselves are also conveniently cast this way. What Necromancy actually entails, how the empire views it and why people do it all stays conveniently obscured.
Granted it makes more sense to spend the dialogue budget (disk space!) on the faction you're playing as these side explanations aren't directly important to the story, it does take away from the over all atmosphere. Something that I think is rather important in a sandbox game.