Why is vinegar not applied to rice used for onigiri like rice for sushi?

Question

So sushi and onigiri (rice balls) are similar to a naive eye to me wherein there's some shaped rice.

Sushi usually gets some vinegar for taste and to help it stick to my understanding.

Yet onigiri doesn't? https://en.wikipedia.org/wiki/Onigiri At most the rice gets plainly salted?

I was wondering if there are particular reasons whether for construction, taste etc for the difference in handling. Both can be eaten hand held so my possible theory of sticky vinegar undesired for onigiri doesn't make sense.

Is it a cultural or traditional reason?

Answer 1

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 "crumble" 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.