But why? Passenger transport is already well catered for and extremely low margin. Also, passengers are annoying and generally not cardboard wrapped. They require seats and toilets and cabin attendants and pretzels and infant flotation devices. Cardboard boxes also don't videotape you when you offload them last minute to make room for something more valuable.

And that's before accounting for scheduling. Freight flights are often late nights, because cardboard boxes don't have eyes that can go red.