The electric potential of earth is taken to be zero because earth is a good

Our earth has its own magnetic field and thus it can be considered as a whole magnet, and we know a good conduct for magnetic field is even a good conductor for electricity. Hence, earth is a conductor.

Electric Potential is given by:

Where, w = work done and q₀ = unit charge.

Since, the size of earth is large and a good conductor. The potential difference of earth remains constant regardless of the electrons taken from it or supplied to it. For this earth is taken to be zero potential surface. Therefore, Earth is good conductor.