I wonder what is happening behind closed doors for these companies to be issuing such a joint letter.

Different companies have different motivations. I'm sure some are genuine, but a lot of them simply lost the race (to OpenAI/Anthropic/Google) and know they can't compete anymore, so are shifting strategies. How many times has Microsoft done exactly this in the past?

If there are effectively only two AI companies allowed to train or run model inference then demand for hosting, GPUs and data centers goes down, so they'd be able to squeeze their suppliers that much more too.