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

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…