I think it was Fred Picker who said that all landscape photographs should be horizontal rectangles because God made people's eyes side by side not one above the other.

The wide aspect ratios like 16:9 seem in practice to be strongly associated with cinema or its home TV/computer incarnations. A while ago I did an informal census of all formats, all media presented to human eyes and by far the most prevalent was "A" format with A4 vertical the commonest size.