The employees haven't left yet. Business continuity is easier to achieve if the employment arrangements don't have to change.

MSFT don't have OpenAI's IP. They have an exclusive right to some of it, but there's presumably a bunch that's not accessible to them. Again, business continuity is easier if they can just grab all of that and keep everything running as normal.

That's a whole lot of training and server infrastructure that would have to be rebuilt - and cloning the exact existing stuff would be one heck of a corporate espionage charge, which I expect Microsoft is very avoidant of.

For the brand, the IP, and the fact it would be pennies on the dollar.

I'm just guessing, but they probably have a restricted license to some IP, not ownership of all title and rights to all IP.

Yep, those lawyers can be just as crafty as developers believe it or not.

*edit: just saw it claimed below Nadella said "we have all of the IP rights to continue the innovation"*

I don't know!