Why does good nigiri often use sushi rice cooled to about body temperature instead of refrigerator-cold rice?

Answer: Keeps starch tender

Keeps starch tender ✓ — Right. Cooked rice is soft because heat gelatinizes starch; when it cools hard, starch chains begin lining back up, which makes rice firmer and less clingy. Body-temperature shari keeps the grains tender enough to merge with fish without becoming hot porridge. The counterintuitive part is that sushi rice is not trying to feel fresh-from-the-fridge; it is trying to avoid the same staling physics that makes leftover rice hard.

Kills surface bacteria — Not quite. Warmth around skin temperature is far too low to sterilize rice, and rice safety is handled by acidification, time control, and hygiene. Public-health rules focus on pH targets such as below 4.6 or lower, not on body-temperature holding as a kill step. The rice can taste better at that temperature, but it is not safer because it is warm.

Stops vinegar flavor — Almost the opposite. Warm rice can make vinegar seasoning more aromatic, while cold rice mutes aroma and can make the bite feel duller. Sushi chefs usually want vinegar to show up as a light fragrance, not disappear. If rice gets too hot the vinegar balance can also become harsh, so the target is a narrow middle, not simply maximum heat.