Why can't you kill the zombie guys in the abandoned dungeon with mortal sword

After beating the game a couple times I realized I never did the weird doctor guys quest so I went to go do it. While I was down there I remembered the zombies and wanted to see if I could kill them with the mortal blade without them getting back up and nothing worked. Is there a reason or just a weird thing. (Also is it the same thing for the people in mibu village?)