It is not easy to have the bar so far behind the neck. This lower position shortens the lever arm formed by the weight of the bar transmitted down the back to the hips, thus producing less torque at the low back and consequently leads to a safer workout. Your shoulders need to be flexible enough to assume this position though.
Notice this guy was a bit hunched at the start of the squat. During the ascend out of the hole, his hips rose faster than the shoulders, which is dangerous and risky.