Except it doesn't and they don't (and aren't).
You don't even have click on short version of the study that the article is reporting (much less pay $20 to download a copy of the whole thing). The information is right there in the article:
Liberal arts graduates don’t fare quite as well when they possess just an undergraduate degree, though. The workers with advanced degrees in any field of study – who make up about 40 percent of all liberal arts graduates, and earn about $20,000 a year more for it -- push the earnings averages up significantly. Among graduates with a baccalaureate degree only, those with humanities and social sciences degrees consistently earn less than anyone else, peaking at about $58,000 a year.Got that? Liberal arts graduates who go on to further study do better than non-liberal arts graduates. Liberal arts graduates who make their way through life solely on the basis of that BA/BS "consistently earn less than anyone else." Isn't that pretty much what everyone thinks anyway? And now there's a study to back it up.
There are good arguments to be made for the value of a liberal arts undergraduate degree. Misleading headlines like this one undermine them all.