The increase in economic value of a BA over a high school diploma is greatest among those least likely to go to college.
Frankly this is obvious. It's like preschool. The incremental educational value of a good preschool is greater for poorer kids than richer kids, because richer kids get many of the elements of preschool at home, such as talking and reading to them.
In this case, those kids who fit the profile of people going to go to college, but who don't go, still have all the advantages of the profile: having parents and friends who went to college, attending a good high school, and so on. Some kids put in applications at fast food and retail. Others ask their dad's friend for a job.