They did have the technology but then who would fix the repair drones?...It could be that the 'scavs' would just shoot down a drone and bait a repair drone so they can ambush it and shoot that down...and so on. If you have a human being, he can get killed and replaced much more easily I think than building a new repair drone (genetics takes care of engineering and growth as opposed to getting supplies and manufacturing each drone.
Also, humans can heal over time if they are injured. They can reason and make plans and use tactics to engage the enemy instead of just flying in guns blazing. And they don't need to 'scan' a location to know somebody's there, their recognition of hostiles is instantaneous and doesn't blatantly give them away in their environment (like the drone's bright scanning light).