>>pknerd+(OP)
Because OpenAI has first mover advantage, an actual product, the household "ChatGPT" name and starting anything from 0 would mean you start with a 3-0 disadvantage. Even if you threw a few billion dollars at it and attracted all of the top minds for at least a year - probably longer - you will be seen as the "ChatGPT alternative". How long before you can capture the momentum they have now with OpenAI? It's also a legal minefield, even when the majority of employees of OpenAI migrate, there will be all kinds of no-competes and conflicts of interest.
Strategically, this is probably a better move. Microsoft doesn't see their investment implode and they probably have some sort of plan to inject or absorb Sam and/or Microsoft back into OpenAI to prevent this in the future. Perhaps replacing the board of directors to prevent further infighting.