And here's why:
http://www.washingtonpost.com/wp-dyn/content/article/2009/01/31/AR2009013101717.html
"Individual Marine combat loads -- including protective gear, weapons, ammunition, water, food and communications gear -- range from 97 to 135 pounds, well over the recommended 50 pounds, a 2007 Navy study found.
In Afghanistan, soldiers routinely carry loads of 130 to 150 pounds for three-day missions, said Jim Stone, acting director of the soldier requirements division at the Army Infantry Center at Fort Benning, Ga. In Iraq, where patrols are more likely to use vehicles, loads range from 60 to nearly 100 pounds, he said. "
The funny part is that the recommendations, and the soldiers actual load, have both remained fairly constant ever since Marius reorganized the Roman legions, probably longer if we had the full record.
What other choice is there in many cases ? Infantry are often sent into an area on foot because there is no other way to get there.
Many times air resupply may not be possible, pack mules would require even more support, the only place left to pile the supplies and equipment needed is on the soldiers back.
That's why their called "Grunt's", it used to be "Marius's mules".
