Smiling to cameras is a learned behavior, and not something unfamiliar adults do naturally. (Kids do more smiling in general and are less aware of cameras / less self-conscious about their appearance, so if you go somewhere without a culture of cameras and take candid shots, you’ll get more smiling kid pictures than smiling adult pictures.

My godparents are indigenous Maya peasants from southern Mexico, and until the last decade or two no adult in their village would ever smile to a camera, and certainly not in a formal portrait. Nowadays they are more exposed to mainstream Mexican/World culture, and norms are changing fast, but if you tell some adult you want to take their portrait you are still likely to get a very serious expression.