It's omitted more often than not because very few (I would have said nobody, but the lovely AnnaS says otherwise) heard it. I would submit that, expecting it, the normal ear would mishear its being there rather than mishear its absence. Playing with it on my tongue, I can hear it go missing through understress, but I can also hear it go missing through awe and self-consciousness. Those who did not hear it at the time were pretty consistent in their commenting on its absence, so either it wasn't spoken or it was lost in transmission noise. Perhaps there is a sound file available somewhere.