>>2229602The 'why' is that the magic wand is simply the wrong way to render images with poorly defined edges. You can grow, feather, and blur your selection, but it will never look as good as just cleanly rendering it.
>>2229603Very nice.
Pic related is a sort of 'best-case-scenario' for a wand render that's cleaned up by hand: it's never going to look as good as a proper render and probably takes longer, and I really only did this to illustrate the limitations of the wand method.
For comparison here's your render on the same BG:
https://files.catbox.moe/bfd2cb.jpg