It might have been the only deal on the table. Perhaps they thought the risk was worth it - good processes don't always lead to good outcomes. Perhaps they felt that the rights they gained to the GPT models was worth it even if they don't get direct influence over OpenAI.
Between Bing, o365, etc. etc. etc. it's possibly they could recoup all of the value of their investment and more. At the very least it is a significant minimization of the downside.