Exos were created in the Golden Age as War Machines, then their use was apparently discontinued after some time. They've only been "recently" recommissioned (since the collapse) by Ghosts making Guardians and by spontaneously 'waking up'. This raise two interesting points: 1) Either Exos were fashioned to have their own unique conscious identity (I.e. whatever constituent medium that's used to fashion their 'mind' evolves a personality); or a 'mind' is placed in them--like a receptacle. 2) The inferred condition that Exo 'corpses' were just laying around, inanimate and empty since the collapse. The 'putting consciousness into Exos' proposition is suggested numerous times in the Grimoire. As are the constituent mediums through which consciousness can be maintained (ex: "biological...circuits...thoughtful-light"). Suggesting that Exos were used as an alternative body for humans or neo-humans to occupy for the purpose of war (there's even some first-hand accounts of Exos speculating this very proposition). Without veering too-far off-topic: Exo-booty exists because human minds were (more than likely) the originally intended impetus for Exo bodies. ...and what human doesn't appreciate a fine booty?