"Agencies cite any number of reasons for keeping names out of the press: allowing lower-level officials to be quoted might steal the spotlight from the Cabinet secretary or other high-ranking official; the briefers are policy wonks who are uncomfortable talking to reporters; the agency is involved in an issue, but in a supporting role; the officials are there to provide context or technical explanations as a courtesy, not to be the face of an agency."My dyspeptic take: What much of this boils down to is that agency heads are ignorant bigshots. They don't know enough to be talking in detail to reporters and they want all the glory they can get. (It's a truism that the first thing any Beltway type does when picking up a book is to look in the index to find his or her name.) "Heads" want to be the "face" of the agency, not the brains, to be "the man".
But putting on my pseudo-economist glasses: any backgrounder involves a quid pro quo. The reporter likes it because they don't display their ignorance, as they might have to do in an open press conference. The official likes it because their hard-earned knowledge, won by years of toil in the trenches, can at least be flaunted. The agency head will tolerate the backgrounders as long as they don't take away any glory or raise questions about the head. There's also a question of balance--if there are more reporters with more time/space to fill with stories than there are agency heads with knowledge to impart, the reporter goes down the food chain.