The savings on maintenance and repairs over the life of the facilities are, generally, much less than the difference in initial construction costs.

There are also maintenance costs associated with buried infrastructure that you are not acknowledging. Drastic changes in temperature, tree roots, flooding, damage from animals, damage from vehicles, etc, all cause damaged conduit and/or damaged cable, which is usually costly and disruptive to repair. Above-ground facilities are obviously way more exposed and therefore damaged more often, but the repairs are generally relatively inexpensive and simple compared to repairs of UG facilities.