No sane movie producer would ever allow a 5 hour movie to happen.
The very fact that they can charge two tickets to see 5 hours of movie instead of one ticket is reason enough to split the movie up into two parts.
But also, as others have pointed out, 5 hours of movies is a long time. It would work out to at least 6 hours of screen time allocated at the theaters, to account for an intermission, previews, cleaning and setup. That's about 2-3 movies a day. If the first showing is at 10:30A, the second showing would be at 4:30P, and the last being at 10:30P. Which means the theaters would have to pay their employees from 10:30A to 4:30A for three showings.
Then there's all the T.V. spots later. If the movie were just one big five hour movie, than T.V. stations wouldn't have to pay as much for the entire content.
DVDs are more expensive, and thus make producers more cash, if there's two instead of one.
Also, all the side merchandise would sell better if there's more hype. And having a story hang on for an entire extra year is a great way to add more hype.
You're right, "they" could make one big five hour movie. But "they" won't.
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