The weird requirement that everyone smile at cameras and photographers only take pictures of smiling people is a relatively recent cultural convention, started in the US and then spread around the world by US influence. I think it has something to do with the culture of photography used for advertising/sales.

If you try taking pictures of very young children who haven’t yet been indoctrinated into the smile-for-the-camera tradition, they’ll make a wide variety of facial expressions for the camera. But American (& al.) children learn very young that the cultural norm is to always make sure to smile when a camera is pointed at you. Even if the children don’t figure this out for themselves (and they usually do), parents and other adults are often quick to tell them to smile or scold them for not smiling.

If you look at pictures of people in posed pictures in the 19th century or in parts of the world where there are few cameras/photographs, the standard expression is usually a serious/formal one. And if you look at candid pictures, people will just be making whatever natural expression they had.