Perhaps many old people have bills to pay and since they probably don't have any retirement benefits anymore they need to go back to work?
I completely don't understand why they are CEOs and crap though, maybe it's a pride thing. "They can have my job when they pry it from my cold dead hands." Or something like that.
I worked at a machine shop where the owner was I believe 85 or so years old and he still worked there full time. The guy obviously had a short temper with me, I don't think he grasped the understanding that I'd never worked at a machine shop before.
I asked one of my coworkers why he still works there, they said he retired a while back but got bored of it and came back to work. WTF.