Because they have no idea how to teach lag pressure. If they spent an hour with a wet mop and put their mind in #3 pressure point, they would realize relatively quickly that laying the shaft flatter at he transition is not the only or best way to put lag pressure on the club.
Why do you think that so many golf digest instructors teaching the steep to flat move with the shaft?
A desire to be on the Elbow Plane through impact aside.......given a plane angle at Top that is higher than a Turned Shoulder Plane, any movement of the Right Shoulder towards the ball in Startdown will take the Hands out. This isnt " Hitting from the top" but it is "over the top", steep. Hitting from the top would have you starting down with the Hands. A reversed order to the 6-M-1 Downstroke Sequence.
To rectify this predicament, whilst maintaining the proper ground up Downstroke Sequence, you must Drop your Hands and Arms from that high plane angle nearly vertically in transition. Vertical drop as its sometimes referred to. This is plane shifting , probably two of them actually.
Homer recommended we avoid the shifting by staying on the Turned Shoulder Plane. During the Backstroke the Hands seek out the Plane Angle that runs through the Turned Right Shoulder. At top the Hands and Right Shoulder are thereby on the same Inclined Plane together and so the movement of the Right Shoulder in Startdown takes the Hands down the plane.
There's a range of Turned Shoulder Plane angles.........they arent particularly "flat" so to speak but the flattest one you can establish is the one that most likely best matches your shaft plane at Address. Meaning that if you do need to shift down it wont be a very big shift. Hence the Standard Shoulder Turn, Flat Back and Down Plane. Weird but wonderful. You'll need some Axis Tilt to pull it off. A Hip Slide. Ideally with a delayed Hip Turn.
Its an amazing pattern. Simple , beautiful. Homer's genius in full bloom.