Interestingly the data shows otherwise. Factors such as parents' education, where one lives, age, race, family structure, IQ are measured and factored in. Two stats techniques are commonly applied. Ordinary Least Squares (OLS) is more traditional while Instrument Variables (IV) is a newer approach which attempts to account for statistical errors such as omitted and unknown variables in order to more strongly determine causality.
If you like stats check out this review of the literature: http://emlab.berkeley.edu/users/card/papers/return-to-school... which concludes that studies "Taken as a whole, however, the findings from the recent IV literature are remarkably consistent with Griliches' (1977) assessment of a much earlier set of studies, and point to a causal effect of education that is as big or bigger than the OLS estimated return, at least for people whose schooling choices are affected by the supply-side innovations that have been studied so far."
There are real economic reasons why governments invest in their citizens' schooling. It's not just because it sounds like a nice idea. The increased income potential of someone going to school pays off for a government higher earnings which is a primary indicator of an economy's health.
I would classify those as luck factors. They don't really address what I was speaking to.
Even if you believe college counts, someone with the fanciest of PhDs is going to really struggle to out-earn high school dropouts if he or she is not willing to work on the job. And that is the point. College is not the driver, it is the person. A person can leverage college to find themselves in better standing, but there are a million other ways to do the same.
Just blindly going to college is not going to improve anything with respect to income. You have to be someone who is cut out to earn more, and if you are that person, you will find a way, college or not.
And I there are many hard working shop floor workers, but they never make it to management. Just working hard on it's own does not earn you more money.
If everyone in the world studied the exact same subject in college, incomes would remain the same as if those people had not gone to college at all. The market forces would remain unchanged.
What I mean by work is to work to set yourself apart. Maybe you need to hit the gym to become unique that way. Maybe you need to work on your singing skills to become unique that way. College can also bring you unique attributes, but it is no more special than any other avenue, at least with respect to income.
Again, college has nothing to do with it on a macro scale. I won't deny anecdotal instances can use college to their advantage. It definitely happens. Random degree holder is not going to be any better off though.
Some people work to live and will only do the bare minimum to get by in order to fulfill their real joys. Those people are less likely to go to college, and are less likely to earn high incomes, because the passion isn't there.
Others live to work. These people love what they do. They go to college because they love it. Then they work really hard to be the best when they enter the workforce, because they love it. The money follows to these people, because they've worked really hard to earn it.
If you forced someone in the former group to go through college, their income coming out is not going to rise. They are still not going to put forth the what it takes to be a higher earner. It's not in their nature, and that's okay.
As they say, correlation does not imply causation. Your earning potential is essentially given to you at birth, plus the luck factor. On a macro level, college isn't going to have any effect on your earnings.