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.
The reason I heard is that it took a long time to take a shot and was expensive. So you did not want to mess up a shoot with a smile. Better go for the safe option and put on your RBF.
When traveling in SE Asia maybe 20 years ago, locals would ask me to have their picture taken. The older people would just stand there stiff as a plank and absolutely no expression on their face.
Younger kids would put their index finger & thumb in the shape of a gun and hold that under their chin and they would smile.
When I take pictures of my own kids (toddlers) they also have no idea about smiling, you have to "teach them" but then you end up with super fake expressions.
Maybe cameras of the time were too big and serious looking, and everyone ended up posing.