The reason birds do not get electrocuted on power lines is because the bird is not creating a circuit and they are not creating a path to the ground.
In order to get electrocuted from touching a power line you must be in contact with the ground and the hot wire.
Or you must be touching both the hot side and the neutral or ground side of the power line.
When a bird is sitting on top of a power line they are not touching the ground and are not grounded out or making contact with the neutral wire so they don't create a circuit so the power and electricity does not go through the bird.
Even a human could sit on top of a live power line and not get electrocuted as long as they are not touching the ground or the neutral wire.
Birds sitting on a wire don't touch the ground (or anything in contact with the ground), so electricity stays in the power line.
But, if a bird touches a power line and equipment or other metal that is grounded, it gives electricity a path to the ground, and the bird could be shocked.
There is no path for current to flow through the earth back to the source. so current does not flow. or if you believe there is a path just because the negative terminal is connected to earth and the end point of your circuit is connected to earth then the answer is still no because the resistance is too high.
Electricity (whether it comes from lightning or any other source) heads to the ground as a result of some very basic forces.
Basically, clouds filled with tons of negatively charged particles are attracted to the positively charged ground.