Cereal is not technically a soup, although it seems like a soup.
Unlike soup, cereal is not a primarily savory dish and is not cooked with stock, is not often served hot and is generally considered to be a breakfast food, instead of a lunch or dinner dish.
Although both soup and cereal are liquid based with solids and also served with a spoon, the ingredients, cultural context and preparation also differ.
Soups are overwhelmingly savory and cereal is often sweet and considered to be a breakfast food and not a main course for dinner or lunch.
Soups are also made with fish, meat or vegetable stock, which is a liquid the other ingredients are cooked in.
The milk in cereal is also not a stock and does not serve the same purpose of creating a flavorful base.
Although some people argue that cereal pieces are cooked before they are packaged, but the final dish of cereal with milk isn't simmered for hours like a soup to meld the flavors together.
Cereal is also a widely accepted breakfast food, which is a cultural association that sets cereal apart from soup, which is mainly eaten for lunch or for dinner.
And some people argue that cereal could be considered a cold soup or type of dessert soup, as some types of soups are served cold and sweet.
And the milk itself is also prepared or pasteurized, which can be seen as a form of broth like infusion into the solids of the cereal.