well actually there are three regions, in first region there is no radiative power only stored(reactive) energy in second region there are both and radiative power starts dominating reactive power, and in far field (fraunhofer region) there is only radiative power, at source the wave is not pure plane wave but as we go further they spread and recieving anteena is in small region as compared to full span of wave so we assume it to be planar, that happens in far field, last but not least if you remember the solution of helmholts equation is based on plane wave assumption and plane wave is in far field only thats why we always consider far field. (oops quite leangthy explanation)