Almost all ocean freight moves in boxes of the same few dimensions, and those dimensions have barely moved in decades. The reason is that the container is the least important part of the system it defines.
The box is an interface, not a product
A container's value comes from being handled identically by a crane in one port, a railcar inland and a road trailer at the far end, without unpacking at any stage.
That only works if the corner fittings, the length and the lifting points are identical, because every machine that touches the box was engineered around those dimensions.
Standardisation moved the industry away from loose cargo handled piece by piece, and the gain came from the uniformity itself rather than from any property of the box.
The surrounding equipment sets the constraint
Ship cell guides, crane spreaders, chassis, rail wells and yard stacking equipment are all built to the standard, and each has a service life measured in decades.
Changing the container would require replacing that installed base worldwide more or less simultaneously, since a port that handled only the new size would be unusable for existing traffic.
The cost of the transition falls on infrastructure owners while the benefit would accrue to shippers, which is why proposals to revise the dimensions have gone nowhere.
Corner castings do most of the work
The steel fittings at the eight corners are what cranes grip, what lashing rods attach to and what transfers the weight of stacked boxes down through the pile.
Because loads pass through those points, a container's strength is concentrated at the corners rather than distributed across the walls, which is why the sides are relatively thin.
Twistlocks engaging those castings secure boxes to each other and to the deck, and the same fitting works whether the container is full, empty or refrigerated.
Variation happens within the envelope
The external dimensions are fixed, but what sits inside them is not. Refrigerated units, tanks, open tops and flat racks all share the same footprint.
Taller variants exist, adding height while keeping length and corner positions unchanged, which lets them use the same handling equipment with only clearance to consider.
That is the general pattern. Innovation happens inside the standard envelope, because anything that breaks the envelope loses access to the network.
Repositioning empties is the standing cost
Trade flows are unbalanced, so boxes accumulate where goods are consumed and run short where goods are made, and empties have to be moved back at the carrier's expense.
Uniformity helps here too, since any empty box of a given size substitutes for any other, which would be impossible with bespoke equipment.
The imbalance is a permanent feature rather than a disruption, and the cost of correcting it is embedded in freight rates on the busier direction.