I don't know how to actually put a title to this one here. But, the question is,
Why do those high altitude porters haul heavy stuff (pretty heavy) on their back and with support by means of a strap on forehead? Doesn't that hurt the neck?
Yes, tumplines do have advantages over a regular backpack, and that includes being able to carry more weight.
African women have been found to carry loads of up to 60 percent of their body weight on their heads more economically than army recruits with a backpack of an equivalent weight. Nepalese porters with a tumpline have been found to be 60 percent faster and 39 percent more powerful than their clients carrying modern packs.
Method rather than fitness level explains why. When used properly, tumplines evenly channel weight down the strongest part of the body. They require good posture and don’t allow for the sloppiness that can be hidden with a hip belt and shoulder straps. They also don’t restrict lung expansion in the way that pack straps can, allowing for deeper and more even breathing, something that is especially important at high altitudes where climbers are at risk of hypoxia.
Yvon Chouinard who is one of the most famous American climbers/mountaineers/ uses a tumpline.
On an expedition to Tibet I turned on a few friends to the tumpline and, whether they had back problems or not, they felt less tired at the end of the day. They noted the biggest advantage at high altitudes, where lung expansion is totally unrestricted. You can breathe with a smoother rhythm and breathe deeper without shoulder straps.
I will note that the sources I read say that it takes some time to get used to and has to be done properly, but they do allow one to carry heavier packs.