they teach you that because its the easier/biggest target to hit. shooting a headshot does not leave much room for error. you hit the front or the back, like the guy in the video explains because the skull thickness.
if you shoot in the typical shoulder/heart area, you're chances of hitting either shoulder, lungs, or heart is so much higher. if you hit only the shoulder, it wont run far. if you only hit the heart, it will bleed out fast. if you only hit the lungs, it will suffocate but run a fair distance.
you hit a combination of the two and that deer isnt going to go far..
i've been hunting since i was 12...