Maybe people just prefer faces to backs of heads.

Interesting but not true. We pose because we want to look a certain way - a harmless vanity of smiling, presenting what we think is our best put-together appearance. Portrait and fashion photographers choreograph the poses of their subjects as well, as part of the process of creating an image that meets the expectations and needs of the audience. There's also the need to light the subject according to aesthetics, and because the lights don't move with the subject, we hold the subject still in relationship to the light. Another aesthetic choice relates to depth-of-field and the in-focus/out-of-focus planes, and again that's something that's easier done with the subject is still.

One could argue that certain static portrait poses are distantly derived from the look of portraits produced in the early days of photography, and that look is a cultural expectation. There's plenty of portrait photography that does not replicate 19th century stylistic elements.

There's also portrait painting which predates any kind of photography by a LONG time and didn't have the necessary technological limitations to be frozen beyond "just let me look at you from a consistent angle again" periodically, yet still has "poses" for the sake of style/messaging/whatever.

That's a good point. Early photography absolutely copied the aesthetics of painting, to the point where painters like Édouard Manet decided maybe realism wasn't all that.

As a photo viewer, I for sure want the people posed for most group photos. I want to see their full faces. That doesn't happen for candid group shots.

If the flash isn't on you can't really tell when the photographer actually takes a photo. I kinda miss that little light point & shoots would have.

It's not really a cultural expectation, it takes some time for the photographer to actually hit the shutter so people who know they are about to be photographed freeze themselves to allow the photographer to get a "good" shot. It's not like if photographs were always instantaneous, there would only be candid shots of people not posing…