I think they use a rope harness that keeps you from going down too far, and then they basically need to come dig you out if you do get pulled it.
That's what I was implying with the sewer worker comparison, since they don't get an escape hatch, just a harness, rope, and winches.

I'm willing to bet if you flipped to the next page after this diagram, it would say something along the lines of "and that's why you always wear your harness".

My guess would be that a door at bottom wouldn't be able to let out enough material to free someone trapped at the top before it jammed up. When they empty them they only don't jam because something is taking the grain away.