Japan was not the only country making camera lenses in the 1970s and 1980s, but Japanese makers had a very large global market share.

Before World War II, many of the most respected cameras and lenses came from Germany. Japanese companies began by copying and learning from German designs in the late 1930s, then improved rapidly. By the 1950s, Japanese cameras and lenses had become popular worldwide, and by the 1970s–1980s brands such as Canon, Nikon, Pentax, Olympus, and Ricoh were dominant in many markets.

Other countries absolutely made cameras and lenses too. Examples mentioned in the discussion include Germany (Leica, Zeiss, Rollei, Praktica), the Soviet Union/Russia and Ukraine (Zenit, Lomo, Kiev), the USA (Kodak, Polaroid), and Sweden (Hasselblad).

So the reason you see “Made in Japan” so often is not that Japan was the only producer, but that it became one of the world’s biggest and most successful camera-manufacturing centers.