That wouldn't work because people need satisfying points along the way.
Even for TV shows with plots spread over a season, each episode has an arc of an intro that hooks you, development, and then a satisfying conclusion. And we can only hold so many plot details in our head at once, which is why movies and plays generally aren't longer than 2 hours and change.
A 9-12 hr megamovie would be deeply unsatisfying to watch, without the carefully structured resolutions along the way. You need satisfying "stopping points" along the way where a few things get resolved but a few more things keep you watching the next night.
It's the same way fiction books need chapters and symphonies need movements. Episodes are a basic creative organizational principle.