Why is vinegar not applied to rice used for onigiri like rice for sushi?
Not a Japanese chef, but I do homemade nori rolls and onigiri:
The purpose of the vinegar in sushi rice is to flavor it, not to make it more sticky.  If anything, the vinegar makes it less sticky due to adding a little acidity and moisture.  Sushi rice is supposed to be delicate and &quot;crumble&quot; when you bite into it.  In contrast, onigiri rice should be tough and springy, almost like a dough, so you want to maximize stickiness.
For this reason, while sushi rice is specifically cooled to room temperature before being formed, onigiri rice is not; it's best to make the balls with the rice still warm from the cooker. Mixing the vinegar into the rice would cool it, and make it harder to form firm balls that hold up to being stuffed into a lunch sack.