24 frames per second is the slowest that you can film without introducing noticeable flicker. Film stock is expensive and so movie makers want to use as little of it as possible. If they could get away with it they probably would have filmed even slower.
In these days of digital recording you can much more easily and cheaply record at higher frame rates but people have become used to 24 fps and see anything faster as “off”. For example, TV shows recorded on tape are at about 30 fps and people associate this frame rate with cheap production values. It’s called the soap opera effect.
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