This is merely your failure to imagine it. Maybe they enjoy it, maybe they have a specific goal they want to achieve, have some sort of obsession, maybe they feel duty to all the people they lead and relie on them, think they are making the world better so they feel compelled to continue working, are mad with power and enjoy lording over other people. There are many many reasons why one would work after being financially secure, and they're all equally valid as a personal motivator.

And, anyways, I think you say this now, but if you were to get those 10 millions you would probably change your tune. A lot of people would find some other project to dedicate their energy. Especially the kind of people that make stuff happen.