You make it sound easy, but there are relatively few billionaires in the US descended from historically wealthy families [0]. First off, this kind of compounding has only really been possible since the industrial revolution (~200 years). Before that, wealth came primarily from land ownership, which was zero-sum. Second, "no major bad luck" has been pretty hard to come by in most of the world across multi-generational time-scales. The US has had a historically exceptional period for the last 80 years or so, but there is no reason that has to continue. Third, "as long as descendants don't make errors" can also be a significant hurdle. Many Asian countries have a saying: wealth does not last past three generations (not meant to be taken literally, but people say it for a reason).
Also, to start compounding, you have to have enough to compound. Most people instead start off significantly in debt (student debt, mortgage/rent, etc.). Then they try to save enough to be able to raise their children and retire before they become too infirm to work, at which point they start compounding in reverse. All the while, they are competing with everyone else trying to do the same thing for how much they are willing to pay for scarce resources and how little they are willing to receive for their employment. Having your compounding achieve escape velocity is the exception, not the rule, and it has to be that way.
Also, to start compounding, you have to have enough to compound. Most people instead start off significantly in debt (student debt, mortgage/rent, etc.). Then they try to save enough to be able to raise their children and retire before they become too infirm to work, at which point they start compounding in reverse. All the while, they are competing with everyone else trying to do the same thing for how much they are willing to pay for scarce resources and how little they are willing to receive for their employment. Having your compounding achieve escape velocity is the exception, not the rule, and it has to be that way.
[0] https://en.wikipedia.org/wiki/The_Missing_Billionaires