Why are overhead transmission lines not insulated

Overhead transmission lines are not insulated primarily due to three reasons: first and foremost is to reduce the weight of the lines, secondly to save cost and the last is they are at sufficient height to avoid direct contacts. Let us discuss each one of the reasons one by one elaborately.

You might have notices that the cables used for residential wiring as well us laid underground are insulated but overhead transmission lines, in particular, the high voltage transmission lines are not insulated. At the same time, the HV and EHV lines are supported over high lattice steel towers. Here are the reasons behind all these.
 

Reduce weight and cost of the transmission line

The thickness and material of insulation required for the transmission lines depend on the transmission voltage. Higher the voltage thicker shall be the insulation. Hence insulating the lines adds up extra weight. Apart from this, storm and accumulation of ice over the line add up the weight and affect the stability. To avoid this, the lines are left uninsulated. Finally, companies can save a lot of money by overhead conductors.