It is because of the "Rectifications orthographiques du français en 1990". French spelling was reformed and streamlined in 1990. Jeux vidéo was the traditional spelling pre-1990. The 1990 reforms stated that it should be spelt jeux vidéos. Not all of the reforms have come into common usage even now 30 years later. Many dictionaries and French learning tools have not been updated to included the reformed spelling. Sometimes both the reformed spelling and the original spelling of certain words and phrases are equally popular and so exist simultaneously in contemporary French.