[raverbashing]

Because any more time the person remains there is giving information to a potential competitor (and there's no point staying if you don't want to be there anymore)

I think Marissa Meyer gave Google a half hour notice.

[nostrademons]

Marissa resigned by e-mail and just didn't come into work that day.

[watwut]

Especially in the case of high level manager leaving voluntary, I would expect him to leave with proper few weeks notice.

Giving company time to shift responsibilities to other people and all that. Spending day or two explaining things to whoever will take his place.

Leaving right now should be nuclear option if you have no other choice or was treated very badly.

[cookiecaper]

He probably gave notice and this just was developed privately until his time had come. It's wise to do this kind of thing in the background. Subordinates are more likely to question the decisions of someone that they know won't be around much longer, etc.