I'd hardly call studies like this useless, as it is normally part of a large glob of other information collected and processed for various reasons.
This is the sort of information people in the college and other education industries want to know, politicians included.
As for the cause, the slow roll, it's very basic economics.
Consider workers to be a form of good used by businesses.
Exceptions exist, of course, but on the education access the bottom of the barrel are the dropouts, then the high school graduates, then finally various levels of college degree.
But then we got on the kick that more education was a way to raise everybody's income, individually and as a whole.
But it should be obvious that not all jobs benefit from, much less require, a degree.
Back in the day, when less than 10% of the population had degrees, they were highly sought after, it really was a ticket to the good life if one could make it.
Today? Over half the population at least starts college. College ends up being a filter for "better" employees when a high school education would be more than sufficient. It's a rather expensive filter.
For example, when my mother started working as an accountant, her specific field was an associate's degree. When she retired? It's now a master's degree, and mom (now upgraded to a bachelor), complained on a regular basis that half the people still couldn't do the job.
What happened? In going from 40% graduating from high school to over 90%, standards dropped. In going from 7.7% graduating from college, to nearly 40%, standards dropped, and college isn't that helpful to that many workers, not unless we start having trade schools hand out degrees.
Which, given that I have an Associates in Applied Information Technology (USAF enlisted degree), and a Masters in Computer Science, and having seen my brother's schooling in electrical work, I'm not opposed to giving the electricians, plumbers, and such who have attended a trade school that meets requirements a degree in "Applied" whatever. I'll note the applied in my degree screwed me when I went to get my bachelor, because they went from accepting it as the first 2 years to breaking it down and going class by class.