Intelligence is inheritable. Intelligence is correlated with educational attainment and wealth. People tend to marry within their socio-economic group.
Wealthy people who came from nothing are the exception. Even in places with high degrees of social mobility, climbing the social ladder is a multi-generational saga. Maybe a working class family has a child or two who enter the professional class, then maybe their child has the opportunity for their family to finance a company that goes big, like Dell or Microsoft, or maybe break in as an actor or sports figure. Most likely though, their kids also enter the professional class where the family rolls the dice again the next generation.
This is because intelligence is not all inherited, so those above the mean owe some of their intelligence to luck. Children do not inherit their parents' luck, so they revert to the mean _compared to their parents_. The children retain their inherited advantage over people who didn't get the good genes.
Wealthy people who came from nothing are the exception. Even in places with high degrees of social mobility, climbing the social ladder is a multi-generational saga. Maybe a working class family has a child or two who enter the professional class, then maybe their child has the opportunity for their family to finance a company that goes big, like Dell or Microsoft, or maybe break in as an actor or sports figure. Most likely though, their kids also enter the professional class where the family rolls the dice again the next generation.