why and how is the degree of a non-zero constant polynomial zero?

A non zero, constant polynomial f(x) = c or in other words

f(x) = c*x^0 (as x^0 = 1)

degree of a polynomial is represented by the highest power to which x is raised to in the polynomial and so, here the highest power to which x is raised is 0. Hence degree of the polynomial is 0.