The Greeks had no act structure in their plays. The plays had one act. The Romans had five acts. It’s arbitrary. It appeared in plays because of the need to have intermissions. People can’t sit for three hours in a theatre listening to an auditory experience without taking a break or going to the restroom. It appears in television shows because they want to have commercial breaks so they can sell something. None of which has anything to do with story. A two hour feature film shown in a movie theatre is a continuous action. There are no intermissions. It’s one continuous act-less event which revolves around a problem. A much better way to look at a story, when you are creating one, is not through any arbitrary division into acts but through the eyes of that problem, which is the central event and the heart of a great story’s structure.