It’s a good point, especially considering that the UNSC has AI’s that actually exceed the mental capabilities of humans in many ways, you’d think they could make, if not combat “androids” (which would require, complex, presumably expensive robotics, many of which would be destroyed in battle), they could make automated tanks, warplanes, etc.
Now, I do think there are a few mentions here and there of automated devices in the Halo universe, but nothing where they do some kind of full scale operation with them.
Now here are possible reasons not to have them:
They could be prohibitively expensive
-smart AI’s are expensive, it’s not like everyone has one in their Toyota in the year 2552
-even if you just consider automated war machines (not human-like robots), they would still probably more complex and expensive than equivalent human controlled machines. What is more effective for the same cost? One of these AI tanks, or three human controlled tanks, for example?
Remote control may be too unreliable, resulting in expensive losses
-okay suppose instead of having the AI in the warmachines, you had a single AI, like Cortana, controlling many of these things from a ship in orbit?
-well in hostile territory, maybe the inconsistencies in terrain and weather would make this type of control (and don’t forget the AI would need realtime feedback as well) too unreliable, resulting in expensive losses
Security
-risk of hacking: even if you think the Covenant is not capable, the risk is still there, also there’s risk from rebels, mercenaries, and anyone who might be willing to profit from compromising the machines at the cost of the military’s chances for success
-risk of AI disobeying? Maybe. Defecting? Maybe; it happened to the Forerunners.
-risk of capture? Is that better or worse than capturing a human controlled warmachine with or without a human pilot? I dunno.
Think about the kind of drones we have now. They can be affective, but they’re not replacements for our advanaced fighter jets with human pilots. Maybe they will be some day, but right now, if the US were to fight an enemy that had competitive aerial combat abilities, we would not be able to succeed with drones.