Microsoft hasn't signed because unlike the others they're scaling back their datacenter plans... Massively. As in, they've cancelled 1GW of future builds, which is equivalent to the entire compute power of London, AKA the datacenter capital of Europe.
MS has seen the writing on the wall; AI is a nothingburger. And keep in mind, MS basically owns OpenAI, the apparent leaders in the field, and has access to all their tech and IP. If Microsoft are calling bullshit, it's not for a lack of information. They know exactly what Sam Altman is cooking up in secret, and it's clear from their reaction that they know its just a new coat of paint on the same busted crap.
If OpenAI really were quietly working away on AGI, or some magical new version of ChatGPT that solves all its problems, MS would be in the best position to get out ahead and profit from that. They'd be building capacity and power generation like crazy.
This whole bubble is primed to burst.