Just like the main gates (after they added touchpoints) I imagine they use ropes because metal gates or walls give off the wrong feeling in people. You think of a closed mall, subway station, or worse: JAIL instead of Disney World.
Adding to this....
Just like touch points, security lanes open and close frequently depending on time of day and traffic level. It is easier(quicker) to undo a chain or rope than a fence. It would also make it easier for an evacuation if needed.