Have you ever encountered portrait photos? They're orientated vertically because the human form, either head, bust or full body, fits better and excludes distractions.
Vertical videos, if they're focused on a human, work fine for the same reason.
Vertical videos, if they're focused on a human, work fine for the same reason.
I suspect that a still image is also different from video because, without motion, there's no feeling that if the person might move a few inches to one side and go out of frame.