Because, under reforms of the 1950s and then Goldwater-Nichols, winning wars is not its job.
As my friend and mentor Bob Killebrew puts it:
"I would just add that by taking the chiefs out of the strategy business, and making them responsible for building the force, they are no longer responsible for winning wars (or for strategy) but for the maintenance and support of their institutions. I suspect this is at least partly why Gates found such a business-as-usual attitude in the Pentagon, and why you see the SecDef dealing so much with the combatant commanders and so little with the chiefs. They are effectively neutered."
Also, just so's youse have it, here's your last chance to read the review I wrote of Robert Gates's memoir for the New York Times Book Review.