From The Wetware Crisis: the Dead Sea effect:

There is an anti-pattern that I’ve seen in large organizations which I have come to call “the Dead Sea effect”. [...] Many large corporate/government IT shops — and not a few small ones — work like the Dead Sea. New hires are brought in as management deems it necessary. Their qualifications (talent, education, professionalism, experience, skills — TEPES) will tend to vary quite a bit, depending upon current needs, employee departure, the personnel budget, and the general hiring ability of those doing the hiring. All things being equal, the general competency of the IT department should have roughly the same distribution as the incoming hires.

But in my experience, that’s not what happens. Instead, what happens is that the more talented and effective IT engineers are the ones most likely to leave — to evaporate, if you will. They are the ones least likely to put up with the frequent stupidities and workplace problems that plague large organizations; they are also the ones most likely to have other opportunities that they can readily move to.