The films are shipped on the 20 minute reels to make shipping easier. It would be very ungainly to try and hoist around an entire film. Also you can replace a bad or damaged reel much easier than changing out the entire film. Add to the fact that projection booths are usually at the top of a winding narrow staircase and you can see how moving entire prints is not an easy thing. If you are moving a print in the theatre you can move an entire print, If you have help and don’t have far to go. Otherwise most projectionists would break the film onto two 6000 foot reels and move the print that way.
To give you an idea, Sahara was 11,158 feet long, and the print itself weighs 47 pounds. That’s without reels or a case.