Mushroom sure spoils fast. I wish they last longer since I love button mushrooms but the grocery store sold them in huge pack that I cannot eat all of them by myself and they got spoil a couple of day later.
@Qelix Your analogy is too way off. Apples and chicken drumstick are different from onigiri. An onigiri is a meal packed in just a rice ball. Apples and chicken drumstick aren't a meal. You will need other foods beside them like bread to be full and to get enough nutrition. If you want to do an analogy, use a sandwich instead but I guess in this medieval era, they haven't thought of that.
There is a reason rice ball have been used in different cultures as a food for travelling or when working far from home. It was used in the past as army's provision too before the invention of hardtack. It is easy to carry, can be eaten quickly with one hand and last longer than foods like chicken drumstick. My country isn't Japan but in the past, travelers, farmers, woodcutters, etc. always packed rice balls.