The richest people in the U.S. aren't Hapsburgs, Rockefellers, or Kennedys. They largely started out in upper middle class, maybe a little better off than that, and then founded huge empires based on a combination of business savvy and innovative disruption. The Waltons inherited their wealth, but the ones on the richest people list inherited directly from Sam Walton, whose story certainly fits that bill.

"Passed down through generations" makes it seem like there's some sort of grand lineage, and I haven't seen any data to back that up.