You can put tens of thousands of copper lines communications in just a single optical cable.
An optical cable is extremely small, in fact, most of the cable is just mechanical reinforcement.
Tens of thousands of copper fibers does not only means lots of cables with lots of devices that could fail, with lots of interference. It means that if something goes wrong, it it very hard to identify the problem or even to have the space to access the problematic cable.
It also means that what used to take a dedicated culvert now could be done with a single cable. So infrastructure prices go extremely low as you only need to bury a small pipe and you gain a lot of space in old infrastructure.
Copper actually rust if exposed to elements. For it not rusting you need to use things that actually degrade or pollute the environment a lot.
An optical line is as inert as it could be, does not degrade over time, does not have EM interference.
You already have all the necessary optical line infrastructure under roads or in powerlines. The only thing you have to update are the optoelectronics terminals and repeaters.