It actually is stated in the game itself ( the lost archeologist I believe will explain it to you ), and it was working this way in the Alpha, making archers very difficult to play since they only were doing Piercing damage.
Slashing though was working OK I think.
Yes, the game lore provides some info on this, but i actually was thinking more in line with the general fantasy lore about undead, from Frankestein monster onwards.
And on my first playthrough of Icewind dale, long, very long time ago - where i intentionally gimped my whole party by making them all dual class, and not using or investing enough points into "turn undead" skill...
Man - i was running away from any undead screaming.
And the memory just got stuck with me.
Undead while i played like that - were the most terrific, scariest enemy i could encounter.
And that was very refreshing - compared to how cheaply that kind of enemy is usually used.
- Its not just about making some kind of logic according to personal preferences. btw.
Internal logical coherence of the setting, then the general fantasy lore - and value in gameplay itself - are important parts all.
@Hiver
Yeah the zooming out bug or rather the "camera doesn't follow the character" bug ! It's actually kind of nice, isn't it ; and it can lead to some nice large overview. Happens when you click to move ( aka waypoint ) instead of holding down the button and dragging ( aka hack'n'skash ).
I know when i happens

I would just want it enabled as a feature - and not disabled in cases like black cove where it happens... naturally (due to differences in heights or - verticality).
This game is very fit for it -since it is gorgeously designed - and it could be very useful for general navigation.