Why would anyone?
That's what I don't get about these people. I'm not saying they should retire, but they can still keep their professional connections and a semblance of "work" life by sitting on various boards or doing non-profit stuff, and not actually have to deal with the humongous stresses of C-level work.
I mean let's be honest: $186M is enough to last Marrissa's descendants forever and ever. If I were in her position, I would figure out a way to make my exit. It's not like she's the founder of Yahoo so there won't be any emotional or moral attachment.