Not ignorant fella, and a fair question. It isn't mandatory for players over the age of 18 to wear neck guards. They are available and all juniors wear them. Wrist guards are also available for the same reason you're asking about the need for a neck guard to be worn.
Players find neck guards really uncomfortable and restrictive, and choose not to wear them.
I watched and photographed Ice Hockey for over 20 years from the mid 80's until I retired as a pro photographer, I used to make a living from it, I must have attended over 2500 games from internationals at world championships, NHL games in North America, european games and at all sorts of levels in the UK.
I have never witnessed a blade injury to the neck in all that time, occasional cuts to the face or chin from high sticks or being hit by the puck but never a blade. I have a photo library of over 150,000 images, and excluding junior hockey I only remember the odd goalie wearing a neck guard as puck protection more than for a blade.
An NHL goalie, Clint Malarchuk got a similar injury in the late 80's and thankfully survived.
Anyone who thinks that there was any intentional action from Petgrave must be mad, he's skating mid ice and is half checked by another player, his momentum topples him over and his legs are shoulder high, as he goes face down to ice, Johnson unfortunately had turned to mid ice at the same time, a totally freak incident.