Why do the French think the Belgians are idiots?

The French spread a joke, saying, "The Belgians brought French fries to go with mashed potatoes." Accusing Belgians of lacking food culture is also their lack of imagination and flexibility. If all the jokes related to "mental deficiency" were told through the mouth of the French, the protagonist would definitely be replaced by the Belgian. In fact, Belgians may not be stupid, but they are too stubborn.