I believe this goes back to the ancient Shinto belief that all beings are inhabited by kami (gods). Therefore to pierce a food is to pierce the god within. This is also why origami developed more broadly in Japan, because paper cutting (a Chinese artform) would damage the kami whereas folding fortified the kami. That is why much origami is of animals. There is also a belief that all food prepared in Japan is given "as is": a crafted piece given down from a master sushi chef, a wagashi artisan, or a tonkatsu expert. Sliced, shaped and formed as a bespoke morsel for the diner, not to be tampered with, but to be appreciated whole.