Because they are based in Iran and Syria, and those are off limits to our guys. He is absolutely correct that if we had more troops on the borders, we could strangle the insurgency in fairly short order.
It was a mistake to downsize our forces in the first place. I figure if the borders are invisible to them, the borders are invisible to us. Bomb the bombers.
Our weakness encourages them.
I agree about sealing the borders, but we haven't sealed our own borders with Mexico. Weak, weak, weak.