why is the electric potential of earth is taken as zero?
Since we know
Q = C * V.
Here, C is capacity to store charge,
V is potential
Q is charge.
Earth is supposed to have infinite capacitance (To make this point clear, take example os sea. What is its capacitance? Of course, infinity. Adding or removing gallons of water wouldn't affect it)
Q/V = C.
Putting C = infinity = 1/0.
Thus electric potential of the Earth comes out to be 0.