Couple of points, page-2

  1. 23,051 Posts.
    It's all to do with insulation.
    In the air depending on the voltage, the bare wires are strung far enough apart to prevent arcing - similar to lightning, where electricity jumps from one wire to the tower and then to earth.
    In the ground bare wire must be well and truly insulated to prevent the wire earthing itself.
    The higher the voltage the more insulation.
    It is the cost of this insulation that costs the big bucks.
 
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