They can only do what they’re told, over and over again. Without a sense of sight or touch, they can’t know if parts or pallets are where they’re supposed to be. A robot or pick-and-place unit will perform its preprogrammed motion regardless of whether the gripper has successfully picked up a part. If that part fails to get added to the workpiece, the pallet will still be passed to subsequent stations, which will mindlessly add value to a worthless assembly.