Also consider the path: you can pretty much do a great-circle with radio.

A cable is not going to be direct. If it's above ground, it needs some slack on the line to account for pole movement (you don't want your fibre holding two poles together...).

You'll need more slack for your splices to be done at surface level during construction. Splice trays every x meters will add more distance.

Sometimes the poles are overloaded and you need to run down and bury it for a while.

Your tunnels are never really straight. You need to follow existing easements, or get your own. When you do bury, you need to go around various physical obstructions, like rivers, buildings, cities, parks.

And if you want redundancy against back-hoe fade, your latency in those situations will shut you down, perhaps erasing your profits when you're down and your competitors are up.

The cost of running your own line point-to-point would be be monstrous, though I guess you could build in some latency and sell access to non-HFTers.