The route an international flight takes is rarely the shortest one available. It is a chain of permissions, each granted by a different country and each capable of being withdrawn.
Sovereignty extends upward
A state controls the airspace above its land and its territorial waters, and that control is as complete as its authority on the ground. Nothing crosses without consent.
Consent for scheduled service is normally established in advance through agreements between governments, which set out which carriers may fly, on what routes, and under what conditions.
Beyond territorial limits lies international airspace, where no state has sovereignty but where responsibility for managing traffic is still assigned to particular countries by agreement.
Overflight is a separate permission
Landing in a country and merely passing over it are distinct rights, negotiated separately. A carrier may hold one without the other, which constrains how a network can be built.
Airlines and their governments therefore accumulate overflight permissions along a corridor before a long route becomes commercially possible, and a single gap can make the whole routing unworkable.
Because these permissions rest on relations between states rather than on commercial contracts, they can be suspended for reasons that have nothing to do with aviation.
Control is handed off in blocks
Airspace is divided into regions, each managed by a designated authority, and an aircraft is transferred from one to the next as it crosses the boundary between them.
These regions do not align neatly with political borders. Oceanic areas in particular are administered by countries whose coastlines happen to sit nearest the traffic.
Each handoff involves a change of radio frequency, a change of controller and sometimes a change of procedure, which is why crews brief boundaries as distinct events.
Passing through is charged for
States levy route charges on aircraft using their airspace, whether or not the aircraft lands, to fund the control services and infrastructure that make the passage possible.
Charges are typically calculated from distance flown within the region and aircraft weight, so the cheapest routing and the shortest routing are not always the same one.
For countries with heavy traffic overhead and modest domestic aviation, these charges represent a meaningful revenue stream that the state has every reason to protect.
A closure reroutes traffic for years
When a country closes its airspace to particular carriers, flights must detour around it, and the detour is measured in additional flying hours rather than in minutes.
Longer routes burn more fuel, which reduces the payload an aircraft can carry, and they can push a flight past the duty limits that govern how long a crew may work.
The commercial effect falls unevenly, since carriers whose geography lets them keep the direct routing retain an advantage over competitors flying the same city pair the long way.