A large share of seaborne trade passes through a handful of narrow waterways. Their importance is a consequence of distance arithmetic that has not changed in a century.
Detours are measured in weeks, not miles
Avoiding a canal or strait usually means going around an entire continent, adding thousands of nautical miles to a voyage.
At the speeds commercial ships sail, that translates into extra weeks at sea, extra fuel, extra crew time and a delayed return to service for the vessel.
The saving offered by the short route is therefore enormous relative to the transit fee, and traffic concentrates accordingly.
Ship design was fitted to the passages
Canals impose limits on length, beam and draught, and whole classes of vessel were designed to the maximum dimensions a given canal would accept.
That created a feedback loop: because the fleet was built to fit, the route became indispensable, and because the route was indispensable, new ships were built to fit it.
Enlargements of canals ripple through shipbuilding for decades afterwards, as owners order vessels sized to the new limit.
Chokepoints concentrate every kind of risk
A narrow passage is vulnerable to a grounding, a mechanical failure, a conflict on its shores, low water levels or a dispute over transit rights.
Because so much traffic uses it, a blockage lasting days produces a queue that takes far longer to clear, and the delay propagates into ports, warehouses and factories inland.
Insurance prices this directly, and war-risk premiums for a given waterway move sharply when tensions rise nearby.
The alternatives are worse than they appear
Rerouting around a continent is possible but requires more ships to move the same cargo at the same frequency, and the fleet cannot be expanded quickly.
Rail land bridges and overland corridors carry a fraction of the volume and cost considerably more per tonne over long distances.
Arctic routes shorten some voyages seasonally but demand ice-capable vessels, carry higher insurance and lack the ports and rescue infrastructure that established routes have.
Control of a passage is political leverage
Whoever governs the shore of a chokepoint holds influence disproportionate to the size of their economy, because they can regulate, price or disrupt transit.
That has made canal treaties, straits conventions and freedom-of-navigation principles central to maritime law rather than technical footnotes.
It also explains why naval presence clusters around the same few points on the map, decade after decade.