Huh I did not know this...
Why is it that when your an extra in a movie you can't look at the camera?
When we watch a movie, the people in the movie aren't suppose to know that they are in a movie. They are suppose to go about their daily lives like you do. I doubt that you go outside and look at cameras.
If nobody sees your face, you can get in the movie more than once (as nobody will recognize you) and get paid for each time. :)
From your perspective the camera is invisible.