But if a subsea line is 6 fibres, you’d think they’d license one of the 288 strands on an existing run from some less else for less than that cost.
This is easier said than done.
There may not be spare strands on the route in question, or if there is, it may not be available at a reasonable cost. Remember dark fiber is an unregulated asset, nobody can force network owners to play ball.
The available terrestrial routes between the landing stations may be suboptimal.
The route between landing stations may be required to be diverse from terrestrial route.
There's a whole another level of control between owning a full route and leasing some strands in somebody else's cable.
...
And the list goes on.
The fiber isn't, but the poles are. You can force the pole owners to play ball.
For your edification, look up "malicious compliance".