Everybody has their own price!
For people like you, Rog or Mark, you would pick up the empty and carry it down if somebody paid you (let's say) $1000. Well, the Sherpa people have a lower standard of income, and they will pick them up for $2, even if there is some reasonable risk involved.
Once they get the empties down low enough, they can haul them out on a yak. But yaks don't operate well when they get onto solid snow, because they have to graze on grass.
If a yak hauls two man-loads down, the yak owner gets paid the equivalent of two human porters. It's kind of a different economy. No plastic. No ATM.