Screenplays are short-stories
In film school we were taught that movies are not like novels, they are like short stories, because:
- 1 protagonist
- 1 central theme
- concise timeline – a sequence of events that only pertain to the protagonist and theme.
We were taught this is the opposite of what novels do (readers were expected to have longer attention spans back then for "serious" novels). Adapting a novel for film meant cutting and combining characters, dropping themes and subtext, most likely re-writing the whole thing into a much simpler "short story" format.
Full novels were sometimes adapted into "event television", a 2 or 3 night mini-series with a total running time between 3.5 and 7 hours depending on how it was formatted to fit the time slots.
Gone With the Wind was adapted into a 4-hour 2-act film. It was literally divided in two, the curtain came down and there was an intermission. The main character is very different in the 1st and 2nd act so it's a logical break. (A modern adaptation would probably divide the book into two movies, but marketing conventions would demand a trilogy.)
Popular entertainment = mass marketing
There was of course a vast selection of trashy mainstream paperbacks at every grocery store: westerns, romances, thrillers and formula crime stories – the descendants of "pulp" fiction, printed on cheap paper to be disposable and marketed around obvious tropes. Their narrative structure was like an extended short story: 1 protagonist, 1 theme, but with more "fan service" – detectives got in more fights, spies blew up more submarines, romances had more sex. It's what we call "genre fiction" today, although genre has arguably expanded the scope of formula it's still very obviously rooted in formula and tropes as a promise to the reader.
Popular media has a lot of crossover. Did you like the movie? Buy the comicbook. Did you read the book? Go see the movie. Pop-media crossovers are well-funded marketing campaigns. As consumers of media, we get a very distorted idea of what is heavily marketed conflated with what is "popular", not to mention "good".
Mass marketing requires conforming to industry standards
Movies are the length they are to suit maximum showings in one day. If the movies were too long they got cut by the theater owners. Other factors included the expense of printing and shipping each reel, which held a specific amount of time. The size of the reel was limited by the size of the projector, so the industry created "one-reelers" and "two-reelers", but frowned on 20-reelers. 5-hour silent films existed that were "important" highbrow think-pieces; shorter slapstick and melodrama was for low-brow audiences (Guess which made more money). Television came with stricter time constraints. Dramatic beats were tied to commercial breaks. Cinema shifted from storytelling to spectacle, featuring wide screen formats and Technicolor™ – technical innovations which also influenced the kinds of stories being made. None of this has anything to do with crafting a compelling narrative.
This year's pop songs will seem very much like pop songs from 50 years ago if you ignore all the artistic music choices and just focus on how it's structured to fulfill the promise of a pop song that fits within the broadcast radio time-slot. There are other types of music of course, but they don't get played on the radio between commercials.
the 3-Act Screenplay is bs
Any cake can be cut into 3 slices, but that doesn't make it a "3-layer cake". Likewise any story, no matter how disorganized and unfocused, can be divided into a beginning, middle, and end. As long as something happens, the conflict beats can be emphasized or inflated to "magically fit" the extremely uninformative and reductive 3-act format. Most people would agree that the 3-act screenplay is actually 4-acts, which proves my point: any story can be shoehorned to fit a 3-act formula if you decide that's how it should to be described.
The "3-Act Screenplay" is a hoax invented by a guy who wrote a How to Write Screenplays manual. It's a helpful idea for new screenwriters to realize they can use conflict and tension to keep the audience awake, but it's not a formula for writing a good story – hence generic screenplays with invented conflicts that make no sense but hit the dramatic beat on page 17 and 87. It's just a formula for preventing the 100 minutes of filler dialog in a movie from dragging. It's not a "universal rule" for writing, not even for cinema. Bollywood movies are usually 5-acts. TV sit-coms are 1-acts, while 1-hour dramas are typically 4 acts of diminishing lengths, with a final scene as an epilog fitted around commercial breaks.
You are right to be skeptical of all writing formulae
Writing formulas like 3-Act Screenplay and Hero's Journey are the pics and shovels of the Hollywood gold rush. Comparisons between pop media formats are misleading (if useful at all), and one media can easily borrow tropes from another – for better or worse. Calling it "3 Acts" is just a metaphor anyway, unlike Gone With the Wind no curtain comes down and there is no intermission. The terminology isn't accurate or insightful, that's not even what theatrical acts do.
The original 3-point structure is called Freytag's pyramid. It's also "universal" because it is a reductive way of looking at all stories. Any story can be described as fitting any structure, if you are willing to ignore the things that don't fit and add some invented drama beats where required, especially if that structure is as broad and generic as possible to begin with.
Jaws could be a Broadway musical if you made it fit the "3-Curtain Structure" of a Broadway musical (with a full-cast opening number and an "I wanna be somewhere else" wistful solo at the top of the 2nd act…). Is that the formula to tell the story of Jaws? Does it mean Jaws the Musical would actually be any good?