German post uses it for next-day letter delivery. They just put the boxes in bags and use the seatbelts to stap the bags to the seats.
Is that really an unsolved problem in airline business? It can't be that hard to build small containers that fit on the seat and can be fastened with the seat belt system.
Obviously, you won't want to do this for general cargo transport, but for "hey, we have a grounded plane, but transporting air mail gives great rates right now" situations?
So it's not a solved problem on massive scale because there aren't frequent and long worldwide air passenger traffic shutdowns.