Lucille
The original 'who does what and when' will have been the subject of considerable discussion between the flight training department of a manufacturer and the flight test department of the manufacturer and almost certainly overseen and approved by the regulating authority of the country in which the aircraft is produced. Countless hours will have been spent trying out various scenarios in order to establish what is safest and most efficient practice and the least likely to cause any confusion, misunderstanding or ambiguity that could lead to and accident or incident or a product liability lawsuit. Now that manufacturers make 'planes that are in many ways very similar, if not identical, some procedures will be copy-over but the original 'what, when and by whom' will have it's origins in the pre-launch phase of a new model. So, to answer one of your question, 'Why is this so?' it is because engineers, pilots, designers and regulatory authorities say it is so, having carried out exhaustive tests and training, a definitive source would be the manufacturers Flight Operations Manual, (I think Boeing call it the Airplane Manual), but not a company's as companies can and do change manufacturers recommended procedures. If companies instigate changes without clearing these changes with the manufacturer first then they may discover their folly when they find they have gone outside the terms and conditions of the manufacturers product liability cover!
You may get an answer closer to the one you are looking for from the Flight Test forum. Best of luck.