Why are there many more openings than hired employees?

A lot of the economics blogosphere has been quietly discussing this recent column by Peter Orszag, the former director of the Office of Management and Budget where he lays out a bit of a puzzle about the current state of the U.S. job market:

An odd puzzle is taking shape in the labor market: Over the past three years, the number of job openings has risen almost 50 percent, but actual hiring has gone up by less than 5 percent. Companies are advertising a lot more jobs, in other words, but not filling them. 

To get some sense of how significant this is, consider that if, since June 2010, hiring had risen a third as much as advertised jobs have (rather than only a 10th), and nothing else were different, job creation would be roughly 500,000 higher each month, and the unemployment rate would already be back to normal levels.

Orszag lays out four different hypotheses, all of which have been discussed widely since the Great Recession, but does not draw any conclusions. (The column is worth reading in full.) The 4 hypotheses are:

• The classic "skills mismatch" explanation, i.e. "We can't find enough CNC technicians!"
• Low offered wages
• "Internal markets" theory, i.e. companies are finding ways of promoting people from within
• A reduction in "recruiting intensity", i.e. they're advertising, but they aren't really looking  

All these theories have some evidence in the academic economics literature behind them (not that I can evaluate them reliably...)

I wonder if we are seeing a similar pattern in the chemistry world. Certainly (and this is a gut feeling and no more), the ads in the back of a physical copy of C&EN have basically dwindled to perhaps 1 industrial ad every two weeks or so. In that same vein, I can say that the last job ad by a major US-based chemical or pharmaceutical company in C&EN was probably 2 or 3 years ago. (Of course, the online C&EN Jobs database has routinely gotten a smattering of job postings directly from larger corporations.) In that sense, I think that "recruiting intensity" is definitely trending downward since 2008.

Are low offered wages keeping out-of-work chemists on the sidelines? Pfffft. Don't buy it for a second. Same with the "skills mismatch" theory.

I like the "internal markets" theory a lot, even though I have little evidence for it. (The paper that most recently and most prominently mentioned it is here; damned if I understand much of it. The basic idea: labor mobility is way down, because companies are doing more hiring from within.) Here's Orszag on that theory:

The third possible explanation is that the gap between job advertising and new hires reflects the growing use of companies’ “internal” labor markets. A variety of other indicators — including fewer people moving to take new jobs — suggests that companies are often filling openings from within. Many nonetheless advertise such positions externally, which would boost the job-offer rate in the data. The survey counts only jobs filled from outside a company in its statistics on hiring, so the increase in job-offer rates for this reason would not correspond to an increase in hiring rates. 

This possibility doesn’t explain why the gap is wider for smaller businesses, because larger companies have more robust internal labor markets. But it is consistent with anecdotal evidence that external applicants are facing more onerous interview processes and that companies are hiring outside job candidates only slowly and cautiously.

I personally know of one "internal markets" hire, but I'm sure that are more, i.e. permanent interns sllooooowly becoming actual full-time employees, etc...

Readers, what do you think? Which hypothesis matches your experience in job hunting recently?