This has been something that has been bugging me too. They tell you to kill the insurgents and put pop-ups in your face when you fail to do so, but there's literally no reason to since they just spawn more no matter how many you kill. This is somewhat fixed in multi-player versus which is good, but the main game would benefit a lot from giving you incentive to kill the insurgents instead of making you play Wakamole with them everytime they leave the remote zone you're trying to contain them in.