Why do birds stand on power lines and don't get hurt?

Because they aren't in contact with the ground. They are not "grounded". If a person could somehow hang from one of the wires (but not both!), they would not be electrocuted either.

Because they only sit on one line and don't complete a new circuit.

They wear little rubber booties.

To get a shock from a voltage source, you need to complete a circuit. If a bird could touch two different wires at once, or span from the wire to the ground, it would complete a circuit and get a shock.

https://www.youtube.com/shorts/Vu4TYB8ZiDM

Because there's nothing to hurt them. The wires are covered, duh.

For perching, warmth, and hunting

Insulation