There's been some mention of souls with regard to both the Fallen and the Hive if I'm not mistaken, leading me to consider that perhaps Exo's are machines that house the souls of deceased humans, enabling them to continue their guardianship after they themselves have fallen. That or possibly the result of some arcane sacrifice, binding themselves to machine forms to fight unrelentingly for humanity. I'm cool with either! I'll be fine with whatever rationale is given for them.