Now why don,t these lines short out across rain or snow as these would be a connection to ground at some point?
Sometimes they do in REALLY extreme circumstances. (all of my line work experiance was <600 in the military and breif, but some of the other guys can eleborate more.)Like a VERY severe driving rain. Even then it would probably be very rare to get a short past all the dry gaps under the insulators. (All those stacked china hats) Epecially with higher voltage it would probably vaporize it before any short. At lower voltage it's probably more common.
I used to live near a beach, and every night the fog would come in heavy. When it did, the HV lines, (Big fat ones, no idea of what voltage they were) would shoot dancing green sparks down the edges and sides of the insulator racks to the ground line on the cross bar. A fairly load steady hissing all night long. Really creepy like something out of a Frankenstien Movie. One night during a storm they shorted during a horizontal down pour. (not sure if it was the wind that pushed the conductors together, or the rain closed the gap) A great explosion and then dark, a second or two before it all came back on. Looked out side and all the lines were steaming!