As a photographer and someone who loves going through old photo galleries, I know exactly what you mean. People look stern, focused, an article I read long time ago about industrialization's social change claimed these photographed faces show a certain determination, direction many seem to find a missing from contemporary times. I'm immediately drawn to cite The Lonely Crowd by David Riesman and others, a sociological work from the 50s that studied the changing character of Americans at the time. The books premise is that people belong to three cultural types: traditional-directed, whos life is practically same as preceding generations, inner-directed, who were still given a rigid framework but could realize their potential at the same time, and other-directed who like antennas keep watching how others behave and act according. It's not hard to read todays social media addict people in developped economies as the latter, thus why the book still reverbates. The book also offers a fourth archetype the authors believe people should strive for, becoming autonomous, basically like inner-directed but with goals chosen by yourself (can't help but feel a nietzschean Übermensch lurking in there too).

Now what you think of would fit the inner-directed type very much. Although social mobility was on an increasing course, traditionalist values still dominates and people were, generally speaking defined by their class and by their work. Respect and social standing were defined by these: how well you did your job, how well did you fit your class's expected behaviour (or that is, 'performed' above or below). The point is you were defined by your own acts which in turn were diected by an inner "gyroscope". While acts still matter of course, today we are concerned about how others sees us, what their expectations are. We put great value on maintaining attractive facades. And the foremost activity we do is consumption, what we consume is what we are, our choices, our preferred styles ranks us.

So goes the book. But my honest opinion as a photographer? This is not what you see on those old photos. The real reason is plainly simple - taking photos wasn't a casual thing (save for a lucky few). It was an occasion to be immortalized, it was "the' image everyone will see of you, so you couldn't just make silly faces as what would your descendants and others think of you, what kind of person you were? Also there is no reason to think smiling should be the basic expression for posingf for photos! If you look at historical paintings you won't find many smiles either. Smiling had different connotations once, a photography blog I'm reading up for this post says in the Victorian age it was considered drunk-ish (or in case of women whore-ish) to smile on photos. Not to mention the financial side, it wasn't cheap to go to a studio and have photos taken of you - I found data for Hungary in 1910, taking a group photo costed as much a labourers daily income! So its not like you could just take multiple photos of yourselves like that. For especially old photos another big technical reason were exposure times, it was only from about the 1870s that typical times went below a second, so the subjects had to maintan a steady face for seconds beforehand, making smiling an uncanny choice of pose.

Finally I want to add that you can find plenty of old photos with people being casual: Here are some I found going through a single Hungarian archive.