Question

why why a bird sitting on a electric pole or wire don't get a shock

Answer 1

First thing is that the wire has insulation which does not allow current to pass through it. And if the insulation ahs weared away or is not present, the brid sitting on it is not connected to the ground, which is at 0V. The bird's body offers a pretty high resistance, hence current does not flow through the bird. So the bird does not get a shock.
We can also avoid getting shocks provided that we DO NOT TOUCH THE GROUND with bare feet. So if you try to hang from an exposed electric wire with your feet dangling in the air, you also won't get any shock.
Hope I helped!
Thanks!