Insulation is VERY critical Underground . Above ground it's achieved by the distance to an Earth The wires are not Covered. Very high voltages reduce transmission losses and Have Less AMPS but require More distance from an earth to avoid ARCING . There can also be capacitance losses. Wires are cooled by the Air. Transformers are also ( usually through OIL.)
Companies in the game Know the relative costs. As wires heat up they droop lower. Usually in response to AMBIENT temps and sunlight. Not load. Above ground has Lightning Protection these days to protect against damage to household items by Over voltage Houses should have Lightning protection in their Own Right. I Installed about 180 Metres of Underground cable to my House when I built it ( Multi Phase), But I doubt it's worth the cost and trouble. Nev