They die because they are just stupid players. However, I would argue that the best strategy is to have your leader fighting along side your teammates in an offensive matter and giving good call outs with the see-through vision. (Although the leader should be on his toes to avoid flankers) This makes the rounds go by quickly, because having 5 people attack and 1 that knows where all the enemies are(instead of only part of the team without the see-through vision) and so you flatten the other team very easily. Defensive play just doesn't work in gears 3.
See Gears of War HeroClix now...!