Well, I think the main reason is that smiling in photos has become a social norm - it's just what people expect to see. When someone looks at a photo, they instinctively judge whether the person looks happy or friendly, and a smile sends that positive signal immediately.
For example, at work when we take team photos in Shanghai, even if we're not particularly happy at that moment, we'll still smile because it creates a better impression. It's like a visual handshake - you want to appear approachable.
That said, I do notice younger people in China are starting to take more natural, candid photos where they don't always smile, which shows attitudes might be changing a bit. But for now, smiling in photos is still the default for most occasions.