Which people in which movies and which pistols? Check out Why do people always shoot pistols/revolvers with one hand in the movies
Here, for instance, is a picture of US troops being trained to shoot M1911s in WWII:
…which I found on the Forgotten Weapons website, and you’ll see that one-handed is the ‘correct hold’ for the Webley .455″ revolver. Note also, terminological bickerers, that it’s officially a ‘pistol.’
You’ll actually struggle to fire older flintlock and percussion single-shot pistols comfortably two-handed because the grip’s the angle of rake is so steep:
The modern, two-handed stances and grips are just that: modern developments created by conscious efforts at improvement and often derived from practical pistol competition events. The Weaver Stance came along in 1959, and Jeff Cooper had only created the Leatherslap events that gave rise to it earlier that decade. In any setting prior to that, a single-handed grip is probably accurate, and believable for quite a while after that.
Indeed, even today, target shooters still regularly use a one-handed hold, like these chaps in a British Army Rifle Association pistol competition: