Some threads really NEED to stay dead and people revive them. This is what caused necroposting to become a thing in the first place. I think you'd see it differently if it were a 60 page fight between you and Victor from two years ago and somebody comes along and gives their 'input' in some weird, misguided attempt at social bonding or whatever the hell it is these people do.
This is also the reason that some threads get locked. Some threads are horrible beasts that just aren't QUITE worthy of locking but everyone kind of wishes they were.