Recommended reading: "The Real Science Gap" in Miller-McCune magazine. Polemic, a bit hyperbolic (Are postdocs the suckers in science's Ponzi scheme?), but it certainly gets at the problem: the U.S. creates far more young scientists than there are permanent jobs in science.
A few choice excerpts:
The system had a basic flaw that was revealed only gradually, as the expansion of academe slowed in the early 1970s: The system’s central feature — the “self-replicating” professor who produces a steady stream of new Ph.D.s as a byproduct of grant research — had no control over the job prospects for those graduates.
The Academies published another report on the science labor force in 2005, Bridges to Independence: Fostering the Independence of New Researchers in Biological Research... Bridges examined the ominous “crisis of expectation” among the thousands of frustrated young scientists unable to move into suitable career employment. The report was motivated by an alarming fact: The average age of scientists winning their first independent NIH grants... had risen to 42, well past the period widely considered a researcher’s most creative.
The obstacles facing today’s young scientists... are structural features of a system that evolved over a period of 60 years and now meets the needs of major interest groups within the existing structure of law and regulation. Essentially, this system provides a continuing supply of exceptionally skilled labor at artificially low prices, permitting the federal government to finance research at low cost.