Because it is unnecessary to wrap the insulation layer for high voltage, and it also wastes money.
The transmission voltage of high-voltage transmission lines is mostly more than 10000 volts, and the voltage of some lines is as high as 100000 volts and 500000 volts. If such lines are to be insulated, the insulation thickness of ordinary wires is far from enough. The voltage of the electric wire in our home is only 220V. As long as we wrap it with a thin insulating layer to prevent people from contacting with the fire wire, we can prevent electric shock. However, high-voltage equipment is different. People standing at a certain distance next to high-voltage equipment will still cause electric shock even if they do not touch it.
Electric shock caused by high voltage can be divided into step voltage electric shock and high voltage arc electric shock.
Step voltage electric shock refers to that if the high-voltage transmission line falls on the ground and there is current flowing into the ground, the potential near the grounding point will gradually decrease to the distance, so there is a voltage difference between the positions where the two feet stand, and the current flows through the human body to cause electric shock, which is called step voltage electric shock.
High-voltage arc electric shock means that when a person approaches the high-voltage equipment to a certain distance, an arc discharge will occur, that is, you will see a flash of light for a moment, and then the person who gets the electric shock will be knocked down and injured or even killed by the high voltage.