If the investigator finds the board was wrong, it makes the new CEO an enemy of the board. If the investigator finds Sam Altman did bad things, it makes Microsoft look bad and incompetent for hiring him; MS is OpenAI biggest client.
And if, as is likely, the investigator finds some blame here and there, and nothing conclusive, nobody's better off and a lot of time and energy was spent learning nothing.
What good can possibly come from this?
Emmet Shear presumably had very little to do with the prior events, but if he's going to salvage the company, he needs to control the narrative around the governance. That means either publicly justifying the boards actions or changing the board.