There they are, in plain sight taunting you. Cannon fodder for everyone to scope in on. Easy money. Furnished with legs, weapons, armor, the Ceph are not dumb. Their technology is quite advanced. How can they just leave their weakness exposed?
Are they cocky? Arrogant? "Here it is, if you can hit it jarhead".
Do they need it exposed?
Is it just a unique gameplay mechanic? Headshots for CELL, back shots for grunts.
Do you like them? Why are they exposed?