My theory is that employers prefer college grads because they see a college degree chiefly as mark of one's ability to obey and conform. Whatever else you learn in college, you learn to sit still for long periods while appearing to be awake. And whatever else you do in a white collar job, most of the time you'll be sitting and faking attention. Sitting still for hours on end, whether in library carrels or office cubicles, does not come naturally to humans. It must be learned, although no college has yet been honest enough to offer a degree in seat-warming.
Or maybe what attracts employers to college grads is the scent of desperation. Unless your parents are rich, you will walk away from commencement with a debt averaging $20,000 and no health insurance. Employers can safely bet that you will not be a trouble-maker, a whistle-blower or any other form of non-"team-player." You will do anything. You will grovel.