My personal theory is that this is all because Zuckerberg has a rivalry with Elon Musk, who is an AI decelerationist (well, when it's convenient for him) and appears to believe in keeping AI in the control of the few. There was a spat between them a few years ago on Twitter where Musk said Zuckerberg had limited understanding of AI tech, after Zuckerberg called out AI doomerism as stupid.
It's a silly but spooky thought that this or similar interactions may have been the butterfly effect that drove at least one of them to take their company in a drastically different direction.
There's probably all sorts of things that happen for reasons we'll never know. These are both immensely powerful men driven by ego and the idea of leaving a legacy. It's not unreasonable to think one of them might throw around a few billion just to spite the other.
