If the problem were only with Iraqi 'insurgents' they would probably be handling it themselves already.
However, the 'this problem' involves Jordanian, Iranian, Syrian and Saudi terrorists as well.
It's our 'problem' too. That's why we're there.
Once we leave, they can send militias out to patrol the border. Though, we can't even secure our own border. What makes you think we will EVER be able to secure Iraq's border?