Because there is nothing that is normally used in a kitchen that could explode, if it came into contact with an electrical spark. So we don't have to use the special wiring methods that would prevent a spark from occuring in the presence of an explosive atmosphere.
Area classification is based on the likelihood that flammable or combustible conditions occur. Class I for example designates an area where these conditions occur under normal operating conditions. I hope your kitchen doesn't fall into this category See also NEC 500.5