According to John Moran, former Investigator and Expert Witness from Sacramento, CA,  this is done because the AR uses an “in-line” recoil system. The bolt recoils straight back into the stock, which is in-line with the bore. That’s why the sights are so high on AR pattern rifles.

When shooting from a standing or kneeling position, in order for the shooter to get his head into the proper position behind the sights or scope, this is the correct position.

Additionally, this position is taught because it keeps the head level to the ground. The inner ear has fluid filled voids that our bodies use to maintain balance. If a shooter tilts their head when shooting, the body will detect that the head is not level and attempt to compensate. This is an autonomous process. Tilting the head makes it harder to hold still, and part of marksmanship is using the body as a stable (still) platform to support the rifle.