On the question of why some flourish and others do not, I think you're just echoing the point that has persisted through this entire thread.
This is your entire statement. If you're wondering why some are able to flourish while others aren't, getting hung up on the technicalities of a pointless statement is a poor start. We know that people are born under various circumstances ranging from remarkably poor to exceptionally rich, and we know that's the best indicator of future wealth. Yes, they have to inherit the money at some point, but they grow up experiencing the benefits (and drawbacks) of that wealth - whether or not it's in their names.
Health of mother - Advantage. Mother being able to take care of child in the early years - Advantage. Dad present - Advantage. Encouraging, safe home environment - Advantage. Parents that expect and encourage excellence (sports, academic) - Advantage. Never going hungry - Advantage. Going to schools with resources and good teachers - Advantage. Going to good college - Advantage. Parents connections (business and academic) - Advantage.
These advantages compound.
There will be outliers with none or only some of this who still happen to make it. They are not common, and not proof that the system works as intended.