The US famously has a high turn-over rate among its elite rich (just look up the Forbes rich lists from ~30 years ago), and also has a relatively high percentage of earned wealth at the top instead of inherited. One reason for that: in the US, extreme charitable giving is routine among the richest, whereas that's rare in eg Europe and Asia (Europe has far more dynastic wealth via family corporations at the top of its food chain; Western Europe has the highest percentage of rich people who inherited their wealth among major global economic regions).

Typically between 2/3 and 3/4 of all members of the Forbes 400 for example, are self-made. Roughly half of all members in that group originate from the middle class or lower.