A lot of the issue boils down to a gap between college and "career" for the first 2-3 years after college. Employers know that hiring a guy with a physics degree to do an engineering or programming job job will be a money loser for at least a couple years, even if they are pretty darn sure he'll come up to speed and be a major contributor. It is safer to either hire someone who is already trained (and grill them about their possible lack of loyalty), or to get someone with just the right set of skills to minimize the training. After a new guy/gal has some experience there is no assurance that they will stick around. Often you can't get a decent raise, no matter how well you are performing, without jumping ship to another company (a dose of bureaucratic stupidity worth ranting about all by itself).
At-will employment has made this entry level dance crappy for both sides. Everyone knows that they can be let go at any time with nothing guaranteed beyond a cashout of their meager vacation accrual. Employers know that if they sink a large amount into an employee to bring them up on a new or in-demand skill it increases the likelihood that the person can get headhunted away. Stock options and other incentives try to patch this broken relationship by putting some carrots out there, but the young guys usually get very few of those until after they have proven themselves (and a lot of companies has dispensed with them for peons entirely). Other companies know they most incentive plans are crappy and they matching the loss with a hiring bonus and/or sign-on options and still be cheaper and easier than training one of their own (and outsiders are smarter, obviously, than the whiners already sucking at the payroll teat).
It is all pretty perverse. I work in the states for a foreign company, and having a counterpart doing the same job with a very different employment setup regularly makes me question the US system. My counterpart is part of a union, has many more holidays, more vacation, has his hours strictly limited, is not allowed to work at all from home, and cannot be easily laid off (and his college was tuition free).