Ans.

The conductor used in overhead transmission and distribution line is called ACSR(Aluminium Conductor Steel Reinforced) which is the combination of aluminum and steel.

So the weight of the conductor is already high so that would add up to the weight of the line resulting into intense pressure on towers.

Another major reason is that the transmission line is carrying extreme high voltage and it would require a whole lot of thick insulation to absorb the heat produced by such high voltage, hence cost will be high.

Also the chances of anyone touching then are very less because they are at a decent hieght from ground. Atually the transmission lines are insulated by “Air”. The current and the voltage in the 3 phase of  over head lines are calculated to let the air insulate them. You might find the wires around you insulated that’s because they carry low voltage and they are surrounded by many objects to block the air to insulate them, while in this case they are in open air.