For instance, the opportunity for a desirable job may be the result of earlier experience attained, which may be the result of choices about how much time to spend on work versus family or other pursuits.
It gets even more complex when you take this back to the time of birth. Differences among parents represent a huge difference in opportunity for their children. But it's hard to imagine this changing dramatically: parents work very hard to give their children whatever kinds of benefits they possibly can -- social, financial, cultural, educational. If you could ever truly prevent parents from giving their children benefits that not all children have, it would remove almost all of the incentive for most kinds of success, and the consequences would be unimaginable (and probably dystopian).