Pakistan isn't just a smaller, dustier U.S. where people have, on average, better tans. It's structured completely differently, and that makes things complicated.
Our military is completely loyal and obedient to the Executive Branch. Pakistan's military and intelligence services are not; they are rife with divided loyalties are are a political force in their own right that must be negotiated with by the Pakistani government. "Why doesn't Pakistan help us fight the WOT?" is a question that doesn't make sense in this context. Which Pakistan are we talking about? At any given time Pakistan's executive may have only some or little power over the military and intelligence services, never the the population at large, much of whom hate our fucking guts.
In addition, Pakistan's got nukes; something that gives them a hell of a lot of bargaining power, as North Korea proved just recently.
Finally, Pakistan is involved in a number of conflicts at present; a low level civil war, Kashmir, and the Taliban's shinanigans, to name a few. It's a very unstable country, and the U.S. doesn't need another government-less shithole like Iraq, particularly just as it's planning on stirring up yet another major clusterfuck with Iran.
Our sway over a fractionalized Pakistan, given its instability and nuclear capability, is limited at best. On any given day, the best we can probably hope for is that they won't do anything completely fucked up.