Microsoft needs OpenAI to make fundamental breakthroughs; that's the thing Microsoft spent money on, the technology. Their 49% investment probably won't directly pay off anyway, what with all the layers of corporate governance OpenAI has in place.
I don't want to go so far as to say that it was some grand conspiracy orchestrated by Satya and Ilya in a dark room one night, but their interests are pretty aligned; and that clip that keeps getting shared with Sam asking Satya on stage about their relationship with OpenAI, and Satya dry-laughing and failing to answer for a few seconds... why did Sam ask that? Its a really strange thing to ask on a stage like this. Why did Satya laugh, and take so long to answer? Just weird.