I would _guess_ that ousting a CEO AND acquiring the core company at the same time are expensive propositions - I'd also guess that MSFT fully plans to address the leadership issue there (Kotick) but going to give him a year to age out of the newly acquired company and take his golden parachute elsewhere.

Smaller M&A where it's easier to swap the leader (like a startup - which most of us are used to) is MUCH easier/cheaper/faster than swapping out an established CEO of a public company.

They'll do it because he's a liability and they want to make a statement to the new company - but it'll be slow.