Screenwriting is a unique art form that has its own set of rules. One of those rules is that a screenplay should have a limited number of pages. A screenplay's length can vary depending on the genre and the desired pacing, but they typically range between 110-120 pages.
The length of a script is determined by a number of factors, including the length and complexity of the story, how much screen time each character will have, and how long it will take for the audience to get bored.
When movies weren't talking yet, film scripts were usually typed on a single sheet of paper. In the early 20th century, the length of a single sheet was about eight and a half feet. After World War I, when films began to record sound and no longer needed to be typed on such large sheets, the standard length for scripts was changed to four feet, or twelve inches.
Script length is an often-discussed topic in Hollywood. The Hollywood standard for script length is 120-180 pages. This is the length of a script for a film that is most likely to get picked up by a studio. Over the past decade, as more people have been producing content online and as technology has advanced, scripts have been getting shorter and shorter.
Screenplays are meant to be read, so the rules regarding their length is simply there because of that. The screenplay should not be too long for the audience to keep reading it, but also not too short.
Some screenplays also have different lengths as they are meant for different purposes. For example, there are short films and feature-length films - both with their own set of rules and requirements.