It seems to me that the real reason is that the attributes one posses to drive them through college are also valuable attributes in business success.
Someone who has no interest in working hard to attain good grades in high school and therefore does not go to college is, generally speaking, someone who has no interest in working hard to get a good job. However, if you took one of those employed college grads and stripped them of their degree, I think we would find their success in business would remain unchanged. It is the person, not the degree, that leads to employment.
Ultimately, we simply do not have enough information to reach any conclusions from the data.
Employers used to give out aptitude tests all the time, prior to the rise of disparate impact lawsuits. Who knows how much better we could have gotten at testing these sorts of things if colleges hadn't essentially been granted a monopoly on employment psychometrics.
Someone who has no interest in working hard to attain good grades in high school and therefore does not go to college is, generally speaking, someone who has no interest in working hard to get a good job. However, if you took one of those employed college grads and stripped them of their degree, I think we would find their success in business would remain unchanged. It is the person, not the degree, that leads to employment.
Ultimately, we simply do not have enough information to reach any conclusions from the data.