Even if this is true, I think the simple reason might be that one of the five eyes country is US, which is probably the global hub for data and services used throughout the world. Also, Britain being near entrance of Europe from Atlantic, and Australia being near Asia would make economical sense for the cables to take that path.
You are missing the fact that most undersea cables get tapped multiple times. Five Eyes normally inspects the data on land, but enemies will do undersea taps.
While a cable is being tapped, there will be a suspicious change in signal strength, and various signal reflections will tell the cable operators where the tap is. Thats bad for a spy agency who want to remain undetected.
Instead, they break the cable in three points deliberately. The middle point is where they put the tap, and the spy agency will repair it. The points either side are simply so that the cable operators don't know where the tap has been inserted, and have to be repaired by the cable operator. That gets expensive, since it will typically happen 3 or 4 times for a new cable install (3 or 4 countries want access to the data).
Cable repair operations are typically public knowledge (they require specialized ships), so anyone who fancies can crunch the data and see how often a cable breaks in multiple places before being repaired to know how often it's tapped... Mediterranean cables seem to see the most taps.
Please don't make guesses. I'm aware of the tapping process.
> Thats bad for a spy agency who want to remain undetected.
Yes, this is inevitable and it's still extremely more stealth that plugging network taps in somebody's else NOC. Especially if the tapping is done illegally.