Why Close to an antenna, in the reactive near field and the radiating near field (Fresnel region), the relative distribution of energy is still changing as you move outward, so a pattern measured there depends on how far away you are. Beyond a transition distance, commonly estimated as 2D squared divided by wavelength where D is the largest antenna dimension, the wavefront is essentially spherical and the relative pattern shape stays the same while absolute field strength simply falls off as 1/distance. That region is the far field, and it is where gain and radiation patterns are meaningfully specified and where modeling software reports its plots.
Watch out The idea that field strength is constant is backwards: in the far field the field strength keeps decreasing with distance (power density as 1/r squared), and it is only the shape of the pattern, not its magnitude, that stops changing.
Far field = pattern SHAPE frozen, strength still fading. Rough boundary 2D^2 divided by wavelength.