Because you have "restaurantens" in front of "kokk". Whenever you have a possessive in front of the noun, you do not use the definite form of the noun.
Could someone please explain, why do I have to write kattenes, not kattens and restaurantens not restaurantenes?
"Kattens" and "restaurantens" are singular - the cat's, the restaurant's. "Kattenes" and "restaurantenes" are plural - the cats', the restaurants'.