The 3 act and 5 act structure of theatre goes back literally thousands of years. Television and film simply picked up that form and used it, in part because playwrights were the first screenwriters. Advertisements between acts was the obvious way to insert advertising into scripted television. 3 act and 5 act scripts would continue to be the norm even if all advertising suddenly disappeared from the earth.
(1 act and 2 act are also common forms, but much less common in half hour or hour long serial dramas and comedies)
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