The economics are unusual in aerospace: almost all the value sits in the component, so a stillage that saves money on the frame and then lets a part contact bare steel has cost far more than it saved. The design follows from that — cradles and profiled supports rather than flat decks, contact faces chosen for the part surface, and restraint that holds geometry through transport rather than simply resting the part in place.
Awkward geometry is the norm. Panels, skins, ducts and machined parts do not stack, and stacking is exactly what a generic container invites. A-frames and profiled frames hold items on edge or in a defined nest, which keeps parts separated, gives clear access for loading and unloading, and removes the temptation to lay one part on another.
Where a unit is a transport frame between sites rather than a static store, road inputs become part of the specification. Support and restraint have to work in a moving trailer as well as on a shop floor, and the safe way to load it has to be the obvious way.