It is widespread. In other words, your thesis is not supported by facts. There is just nothing to explain here, except how the illusion of self-made fortunes is perpetuated in the teeth of the facts. The US has appallingly low social mobility, and a ever rising share of very rich americans got that way by inheritance. This isn't obvious because flaunting the fact that you got born with a silver spoon for every day of the week in your mouth runs against the american mythos - Members of the British old money crowd are proud of the fact that they personally did nothing to create their wealth and flaunt it. A proper 'murican in the exact same situation is slightly embarrassed by it and might at least show up for board meetings in the family business so that they can maintain some pretense that they work for a living.
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