The history section says it all. There was a fad of agriculture related candies targeted at rural customers. Jelly beans, squash shapes and so on.

Don't forget we also have circus Peanuts, gummy worms, candy ribbons, etc. Many candies have some sort of shape and design to look like non-candy things just for the novelty.

edit: damn, seems legit https://en.wikipedia.org/wiki/Milk