It's true that all of those people did have a lot to give away, but if you had billions of dollars, would you want to part with it? Also, if I remember correctly, Bill Gates convinced dozens of the richest people in the world to give away half of their fortunes.

http://en.wikipedia.org/wiki/The_Giving_Pledge

Would you give away half of your money? No, of course not. Not all of the 1% are greedy money hoarders. 50% is much more than anyone would agree to be taxed... And this was 50% of their total wealth.