That's with a long history of estate taxes and strong economic growth. Also, if you look at Americans with 10+ billion far more than 12% inherited their money.
The US has been stagnating with lower GDP growth rate over time which will likely change these numbers. Because the low % of inhered wealth is mostly due to large numbers of new wealthy not children of the wealthy losing their money. http://www.lagunabeachbikini.com/images/2014/economy/RGDPgro...
88% of the very wealthy ($30+ million) in the USA made their wealth themselves.
https://www.forbes.com/sites/niallmccarthy/2016/10/10/where-...
...the best source I could find.