Lets find out shall we!!?

NOW!!!
The conductor used in overhead lines is called ACSR (Aluminum Conductor Steel Reinforced), which is a combination of aluminum and steel.
So the weight of this ACSR (Aluminum Conductor Steel Reinforced) on their own was already heavy, so it would increase the weight of the transmission cord and put a lot of pressure on the towers if any insulating material is added.
Another important reason is that, you see those transmission lines, they already carry an extremely high voltage and would require a large amount of thick insulation to absorb and withstand the heat generated by such a high voltage, so the cost will be high.
Also, because they are at a decent height above the ground, the chances of someone touching each other are then very slim.
Currently, the transmission lines are insulated by air. The current and voltage in the 3 phases of the overhead lines are calculated in such a way that the air insulates them. You may find the wires you see around you insulated, the only reason for that is because they carry low voltage and are surrounded by many objects which blocks the air that could serve as an insulator, while in most images you see like in this case, they are in naked air.

Thanks for reading, Sayonara 🖐️