It is more complex than that because they cant change what they do on a whim. no-profits have charters and documents of incorporation, which are the rules they will operate by both now and moving forward.
Why do you think that money was spent a decade ago? Open AI wasn't even founded 10 years ago. Musk's funding was the lions share of all funding until the Microsoft deal in 2019
The reality was different. Prior to MSFT, Open AI ran a lean company operating within the the budget of Musk funding, focusing on science and talent. For example, in 2017, their annual compute spend was <$8 million compared to like 450 million for deep mind.
Big spend only came after MSFT, which invested $1B and then $10B, primarily in the form of credit for compute.
He has a competitor now that is not very good, so he is suing to slow them down.