Personaly I think they have a developed such a rich and deep story line that no one in their original target audience would be able to understand it. That's why they are waiting till their audience reaches the new target age group of about 40 something.
It could just be that they are seriously lacking any developement talent.
id have written and released Quake 3+Team Arena and Doom 3 in the same time span. That's two new graphics engines, two sound engines and a physics engine all written by id. On top of that they support countless other dev teams who license id engines and have gone on to produce some of the biggest titles in the last 5 years. ( RtCW / Moh / CoD / Sof I&II / Elite Force I&II to name a few )
Plus how long were crytek going before releasing FarCry ? with it's impressive new engine & physics etc.
DNF & HL2 are the biggest hyped games to ever almost exist. I believe that both will look dated by the time they actually hit store shelfs. ( assuming they make it at all ).