Because it’s not a class group photo, it’s an organization group photo. It’s not common in the US to have a class group photo beyond grade school. (Age 13.) In high schools and especially Universities, the only group photos in yearbooks will be for organizations such the basketball team, school paper, quiz bowl club, Law Review, etc etc...
E.g., The baton is a Harvard Law Review thing, not a law school thing.
HARVARD LAW SCHOOL GROUP IMAGES. FROM THE EARLY 1900’S.