Nobody steals existing copper now. While lifespan is an issue, TCL's HFC network has now been overhead in Wellington and Kapiti for ~15 years and Christchurch for a few years less, and the number of major outages due to overhead cabling has been pretty minimal.
The best outage we had when i was at Saturn was some local decided it was a good idea to shoot a possum of the pole we had fibre on. It was such a fluke shot it went through the cable, it took the techs a while to find the break, because it was not an obvious pole down sort of situation.
We did have copper stolen, but there is no value in stealing fibre off a pole...
Other than that we had very few issues.
Oh, it was about 10x more expensive to build underground, numbers above are about right. And that was for only two ducts, in road, no concrete etc.
People just dont like to look at the cables.