Did you ever notice that when someone gets shot in the back with an arrow, hit with a dagger/knife, or hacked with an axe or hatchet that sticks in them, it’s always right in the spine? Easier for the make-up guys to rig, but how about a shot to the kidney or offset for a lung shot more often?
I suppose it’s to immediately say “This was a fatal wound”. A kidney shot may or may not be immediately fatal but, even if it should be, not everyone is going to say “Oh, axed in the kidney! He’s dead!”
My immediate thought when I see an arrow sticking out of the middle of the guy’s back is, ‘That had to have severed the spine. He must be paralysed from there down.’
My immediate thought when saying that is “The stunt department uses symmetrical padding for their arrow gags.” 
It also reinforces the fact that projectile weapons instructors have emphsized CoM shots since Time began.
There’s a tarot card called ‘Ten of Swords’. Prone dead figure face down impaled with swords, mostly in the spine. Draw that card, and think “well, this can’t be good.” You are right, it’s worse than ‘Death’.
It’s better than an arrow to the knee.