That is a very valid question, Firstly most of the complication are sub assemblies that can be packed individually in a small boxes, likewise the two balance assemblies.
The main clock movement will bolt to a base that forms part of a box. This is, in turn, placed in another box with shock absorbing sponge pads between the two boxes. There can be up to three boxes inside each other. There are sock sensors and tilt sensors that are glued to the box, and, there are fine art shipping companies that are specialist in this type of transport. They supervise the loading and unloading at the airport, as well as customs inspections. Then there is the insurance and the reassembly to think of. It is an expensive operation. It is a tremendous relief when a clock is finally delivered and running. I have been through this process three times , Once there was damage to the wooden pedestal of an antique clock, when a truck driver tipped the pedestal on its side in the truck and it landed hard, there was checker pattern imprint in the timber. Unfortunately this was only wrapped in bubble wrap plastic as it was fairly robust and needed restoration. So there are problems.