Bill Gates is new money, but his father has considerable wealth as well. If those two weren't as committed to philanthropy as your average billionaires are, then the children of Bill and Melinda Gates would inherit several enormous fortunes.
The Koch Brothers inherited from their father, and the five Waltons on the list inherited from theirs. That means over half of the wealthiest individuals in America inherited a considerable fortune.
In forty years this list will no doubt the heirs of other fortunes.
Like Bill Gates, the richest American? Oh wait, he didn't inherit that money. Ooops.