Unlike the edge of the nest-tray, they will never had an adult on the other side of the ledge wall tempting them out with food. They are also predisposed at this age to run away from the edge of the ledge if they can see down (keep in mind they can barely see over) - have you noticed they all sleep with their faces in the wall furthest away from the edge?
The other side of the coin is that if a chick was foolish enough to try to go over, those are perhaps not genes we want in our gene pool - we want birds with a little common sense i.e., peregrines are cliff-dwelling birds, chicks as a rule stay on ledge until they can fly, flying must wait for long brown feathers which 16-day-old chicks do not yet possess.
You can only do so much folks, they are hardwired to stay out of trouble on ledges and they have plenty of room on this ledge that they don't need to hang out at/on the edge.