Short answer: The movie experience isn't close enough to real life.
Long answer: Our bodies are hard wired in certain ways. Ways in which suit us for survival. Eyes in front, ears cupped to the front, arms able to thrust forward, etc.
When we experience real life actions in real time, our body is remarkably good at paying attention to the inputs. I remember being in the school lunchroom when I was a kid and a plate would crash to the ground, not a single kid could resist turning in that direction to see what was going on. Have you ever been near traffic and heard screeching tires coming to an abrupt stop? No way you're not looking. Reacting really.
A movie producer wants your attention on the screen. All the sounds that are needed for your attention are funneled through your speakers in such a way that it appears and sounds "lifelike".
In a full blown theater we typically don't find people falling asleep. Sometimes, but not nearly as much as we hear about it at home and anecdotally our own experience of the phenomenon. This is where the why comes in.
In a big movie theater the screen is crazy large. It's taking up more of our field of vision. Next time you're in a theater, hold your hand out at arms length and hold your thumb and forefinger at an L and make a note of how large the screen is with that measurement as oppose to home. Then the audio is set up with full range sounds. The lows are really low and booming and the highs are crystal clear. Vocals sound like the people are right there. When people are talking on the left side of the screen, it sounds like their voices are coming right from their image. It's cool and lifelike and exciting and enjoyable and.... We pay $12+ to go. An experience.
Let's go home to watch a movie now...
There are a few degrees of home theater:
1. TV with the sound coming from the TV
2. TV with the sound coming from a sound bar and maybe subwoofer.
3. TV with a surround done without optimized speaker placement and subwoofer.
4. TV with an appropriate audio layout
We can also throw in the quality of the speakers as a variable on top of #2 through #4.
Here's what's happening on all of the options below the most optimal (#4 with quality speakers).
Remember when we approached the survival mechanisms of the way our bodies have evolved? Another thing has evolved as well, our brain. Our brain is the CPU of the body computer. It's taking the inputs of the world and making sense of them. In REAL time. It works all the time. It's a hard worker, but it gets tired. Think of the times when you've taken a long drive and you're exhausted when you get there. You didn't move your body around in the car as much as you would have taking a short walk, but your head is tired. What's happening is that your brain has been taking in all of that scenery, all the other traffic, made millions (maybe billions, trillions?!) of corrective decisions while driving, daydreaming along with the song on the radio, yelling at the kids, whew!! That's big brain work.
We get tired at home watching a movie for the same reasons AND this next reason.
You're doing reality wrong.
Not only do our brains need to focus on the screen and what we see, we're listening for clues about what's going on as well. IF IT DOESN'T SOUND REALISTIC OUR BRAINS CORRECT THE INPUT ERROR TO MAKE SENSE OF THE WORLD. This is the crux of it. Our brains are on high alert to make the world make sense based on our experience and evolutionary needs to correctly identify where threats may be coming from. You have no conscious control over this. Your brain is doing it's best all the time. If you try to confuse it, it will fix it. But at the cost of energy.
Example 1. Let's say you have a nice family room with a beautiful fireplace as the center piece but you want to balance the room out and put the TV screen over the top of the mantle. Great setup visually. You also want a theater sound system in there and you know that you need the standard 5 speaker setup (Left front, center, right front, left rear and right rear). All the speakers have a place in that setup except the center channel. The mantle has no place to put it. So you figure you'll put it in the ceiling over the top of the TV. Problem solved. Nope. Problem created.
The center channel, from a movie producer's stand point, is to deliver the sounds that should come directly from the screen. Remember, lifelike. But now they're coming from slightly above the screen. The brain senses this and corrects the problem. You "know" those sounds are coming from the actor/actress on the screen now. But you've just gave your brain about twice as much (I'm not a scientist, it could be 10x but I know it's more) to do unconsciously during those two hours and if you make it through without falling asleep, you'll be more tired than needed. We now make angled ceiling speakers that help minimize this effect.
Example 2. You're working with a large flat wall on one side of the room. Perfectly placed theater seating and all that. The screen is at eye level to the best seat in the house, but you've got some cosmetic reason to put the speakers higher on the wall above the screen. You've created problem for your brain again. It has to constantly correct for placement of the sounds. Even the stuff that's off the screen that comes into scene. Something realistically will sound like it's coming from above and then suddenly appear on screen. There will be a jump of what should be higher, but in your local reality is lower. Brain to the rescue. The brain fixes this so that the world makes sense...but it doesn't.
Example 3. Speaker quality. Speaker accuracy. These issues really matter for a lifelike, enjoyable experience. If you're listening to a compressed soundtrack of screen examples that you know should sound a certain way in real life, your brain is going to correct it. This is REALLY much more important in music. Our brains take on music as waves. Vibrations. We have this magical ability to "know" what vibrations that we know as music should sound like. I'll be writing about this in another blog post as it pertains to emotions elicited from listening to music live vs. recorded vs. recorded badly.
For our experience to be enjoyable and relaxing, it has to come at us in our natural state, without further processing. Only then can we relax, enjoy and be fully awake for it!