Silly noob question, but...like, why aren't revenants undead?

I should probably preface this with that I haven't actually had a chance to play the game yet. I'm just looking at the materials and trying to learn for now. Not to mention, it doesn't really impact my character-designing decisions too much, but the more I think about it the more odd it is.

It being a background instead of a heritage notwithstanding, the only reason I could think of for it not making you undead(or at least having the option) was balance concerns regarding an undead player character. I mean, obviously, right? But...then I remember you can just literally play as a skeleton.
So...what's up with that? Am I the weird one here?