>>340553Because your natural center of gravity is above your hips. Like I said, you still want some of the load above your hips for balance and stability. If your center of gravity is all wonky, it makes it harder for your to keep your balance and move around so once again your body must work harder which means you get tired faster.
Try this out some time if you like. Go to a store that has weight bags for testing backpacks. Get a decent sized pack and load it up with about 30-40 lbs.
First, adjust the pack so that all the weight is on your shoulders and walk around a bit. You'll start to feel your back, neck and shoulders cramp up. You'll start to hunch and lean over.
Next adjust the pack so that all the weight is on the hip belt and the shoulder harness is slack. Walk around for a bit. You'll find that it's a lot harder to keep your balance and move at a normal walking rhythm.
Lastly, adjust the pack to the 70/30 ratio and you'll notice that while you are AWARE of the extra weight you are carrying, you still move and walk like you normally would just perhaps a bit slower. You don't have to exert any extra effort to maintain a comfortable posture or keep your balance.
