You can't pass the Spawn Restricted areas without killing all the mobs in there. So people are forced to linger there, which is what you want. Also, maybe people are trying to complete a bounty rather than farming for loot. (I've been known to speed thru the first two open areas of the earth strike. Generally, if my two teammates grab their sparrows, I'll follow along, but if it's 1-1, I'll stay with the guy who is shooting.)