Note: I've edited the emphasis of this question to address comments' concerns, but I'm only half-satisfied with the change in the title to reflect this.
Alfred Hitchcock took several precautions to protect the twists in his 1960 film Psycho. One was prohibiting people from entering the film after it started (it's been discussed here before). This was an unusual policy for the time, but Hitchcock feared those who admitted late might not see Janet Leigh and feel cheated (Leigh, pp. 96-97, cited here).
Leigh's last appearance is almost at the film's halfway point, so this fear implies a very late expectation for when people would arrive. Why would this be normal for the time, at least in Hitchcock's imagination?
Perhaps Hitchcock misimagined or overestimated a propensity for late arrivals. Failing this, and in the interests of keeping this question on-topic, I'm happy for it to be answered in terms of how film-making in that era either encouraged or responded to this practice (and I imagine they responded to its decline too). Indeed, such explanations would be interesting.
The most mundane explanation I can imagine is that, because cinema visits were much more frequent in that era, people felt the best way to work their schedules around cinema timetables was to arrive late on some occasions and early on others to compensate. (Admittedly, this probably wouldn't explain being that late.) Even so, one would think if they did this they'd prefer to see the two halves of a film in the right order, which only makes the behaviour more puzzling.