Bill Gates, Larry Ellison, Michael Bloomburg and Warren Buffet have pledged to (and already started) give all their money away. Bill Gates has almost single-handedly eradicated polio around the world.

Anyway, none of these people are holding onto cash. They have investments in other companies, which if they just liquidated would, again, cause a lot of damage to those companies, their employees and the other investors. Those companies are doing something helpful to society or else they wouldn't be making money.