The extra length is for car seats for infants. You need that extra length to get leverage as you reaching behind the car seat to pull the belt tight. You would not need as much length to pull it tight over your hips. Also for child booster seats you need extra because of the height added to the seat and the belt has to go on top of the booster seat. In an emergency, that extra length can be helpful. The seat belt used to be and maybe still is attached to an anchor and the part attached is actually the piece that goes into he buckle. That means in a crash and survival situation, you can unmount seat belt and clip several together to make a very long belt to use for survival. There is a purpose and to everything on a plane. Airlines are too cheap and would not pay for more than what is needed or required.  Also, I have seen boxes on the back seats on planes a few times be buckled up so they don't shift.