Here is photo of Tiger Woods and Adam Scott in mid-downswing.
Note that the back of their flat left wrist/hand is against the inclined plane. Note that their clubshaft hosel and clubface sweetspot is also along the same plane.
Now, you stated "why not" - which means that you can imagine that the cluface sweetspot can be on a different plane than the back of the left wrist/hand at this time point.
To me, that means that you really do not understand the relationship between left hand movements and the clubface. If the clubface was not on the same plane as the back of the left wrist/hand at this time point, then the left wrist/hand cannot be flat - it would have to be bent. In my mental universe, the left wrist/hand must always remain flat throughout the entire swing. If the left wrist/hand always remains flat and the clubface is always exactly in a straight line relationship with the back of the flat left wrist/hand, then the clubface must always rotate at the same rpm as the back of the flat left wrist/hand.
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