Buying them (or getting de facto control) is clearly an easier way to achieve that, vs. replicating the technology in-house.
IMO this is the most important part of Nadella's blog post:
> Most importantly, we’re committed to delivering all of this to our customers while building for the future.
It's curious to me that they see the departure of Sam Altman as a reason to remind us that they are "building for the future" (which I take to mean: working toward independence from OpenAI). I think it actually lends credence to the theory that this was a failed power grab of some sort.