I submit it has to do with how much energy they are capable of directing outward, how much new thinking they are capable of doing, how much creative thought, etc.

And what kind of education they get due to where they live due to their parents' income level. And what kind of college education, if any, their parents can help them afford. And whether or not they can leverage any of their parents' acquaintances. And whether they can find a job that allows them to do more than scrape by. And what the economy is doing when they start looking for their first post-school job. And what they look like (however wrong it is, ugly people are less likely to be hired--this has been studied so don't say it ain't so). And how f***ing tall they are! (same deal). And what accent they grew up with. And what color their skin is.

This is NOT NOT NOT a meritocracy. Learn this, memorize it, write it on your foreheads and the back of your wrists. Anybody who tells you otherwise is either denying the advantages they did nothing to gain, or is selling you something.