First off, the US forces in Afghanistan are screwed big time. The supply lines are gone, so they're only going to be supplied by air, through territory with anti-air capability to territory where disruptive elements have the capability to screw with landing zones.
Fighting through Pakistan is going to be messy. It's not good territory for the US to use its advantages in mobility.
China and Pakistan have strong military ties. Good luck with pissing off the Chinese big time. Any attack on Pakistan is going to wreck the ports, which have huge Chinese investments, so you've just pissed off China seriously.
It's also right on the doorstep of Iran, so you're relying on the goodwill of Iran to cover your flank. Or you're going to have to divert a large screening force to protect that flank.
And all this to chase down one person who - amazingly enough - can move.
And that's without talking about the Pakistan disporia.
It's not the smartest idea in the world.