The Tet was big, and it is possible that either the physical size of the AI, or its power requirements, made it impractical to install in the drones.
Perhaps it could remote pilot them, but maybe the delay in communications made them less accurate and effective at their primary killing role.
Further, while it might have been able to remote pilot 1, 5 or ten, does not mean it could control at least 200. I pulled the number out of the air, but given the 1000s of clones seen in the Tet, and the '5 year life span' implies at least 200 actively working Jacks and Vickas. I think 200 is a conservative estimate.
Actually, perhaps I have been thinking too much on this, but the entire movie makes a whole lot more (chilling) sense if we presume that it is actually a Berserker probe.
Let's consider what the priorities of such a probe might be. In order, I'd argue that it would be most effective as:
- Replicate self at greater rate than destroyed.
- Maintain/preserve self structure.
- When meeting another, share information on lessons learned.
- Ensure own technology does not fall into the hands of technological life forms.
- Scan universe for radio signals or evidence of technological life forms.
- Steal whatever technology is developed by technological life forms that might be useful in defeating them.
- Kill all life-forms that are technological.
- Scan universe for planets that contain atmospheres indicating non-technological life.
- Kill all simple life forms.
What we're seeing in Oblivion, is the Berserker implementing point 7 (kill technological life forms).
It does not intend to simply decimate the planet using nuclear weapons, given when it arrived, it had no clear evidence we possessed them. If it uses a neutron (or hydrogen, or conventional nuclear) bomb against us, and (for whatever reason) one of the bombs fails to detonate, we might quickly work out how to make our own, and cause problems for it.
That would violate point 4 (keep own tech.), which will then possibly violate point 2 (preserve self).
It might have gained the ability to replicate life forms based on point 6 (steal technology) after encountering a previous technological life form.
It does not intend to risk the life form making an anti-Tet, so it needs to remain 'out of reach', in orbit).
But the simple probes it sends down are not quite up to the task of self repair (dumbed down tech.), another tech. that it does not risk sharing.
This could explain why the Drones are relatively simple, with no inbuilt A.I.
So the plan for Earth is: Wait till the technological species comes to space. Bait them and capture them. Replicate them into a clone army that wipes out most of the inhabitants.
Once you have them reduced them to only a few percent left, with their technology and production facilities largely in ruins, then risk sending in the simple drones, using clones to repair and maintain them. The 'we need resources' story is actually partly a cover story to firstly convince the clones they need to keep the drones in action, but it also helps to reduce life in general, and make it ever more difficult for the remain technological life-forms to survive.
The last few people might retreat to ..the bottoms of salt mines, or other very secure facilities where they can survive a nuclear bomb, so even then it does not nuke the planet for fear of losing that technology to them. Instead it runs ..4 or 5 generations of Jacks and Vickas to make sure no more were hiding in deep, dark places while it continues to drain resources, putting further pressure on all life forms.
Once it has run those several generations (heck, it's effectively eternal, so it could do that for a thousand years) and is absolutely convinced there are no technological life-forms left, it calls the remaining clones back to the Tet and destroys the last of them.
Then it drains whatever remains of the ocean to ensure all deep sea creatures are accounted for and nukes the planet into 'Oblivion' (pun fully intended).
Of course, during the 'destroy technological life form' phase it is also pulling resource from which it can maintain itself and replicate itself.
So if that's the case, why didn't the Scavs see two Tets, then three, then..?
Again, it is patient. And there is nothing to say that it isn't making them. All it needs to do is make them in the outer Solar system, way from our prying eyes. There actually might be two more Tets already completed, both checking for a 'daily update' from the one in Earth orbit. If they don't get it at the required time, ..send in the next Tet, while the last concentrates on ..making another!
Of course, it does kind of mean that the Human victory was only very temporary. Ooh, a chill just ran down my spine.
But wait, you say. Weren't the resources almost depleted? How would the Tet maintain the cover story to keep the clones going?
I argue that was a complete lie. Most of the land had been laid to waste, giving the impression that the resources were almost depleted. But Jack and Vicka only patrolled a small area that included both New York and was close to the remaining ocean. If the oceans had been almost drained, they'd be stationed close to the Marianas Trench.
But.. couldn't Beacham have discovered a book on geography in his travels, and worked out that there were still plenty of ocean left? Not necessarily. Books seemed scarce, and even if he discovered an atlas, it might have been damaged, with the front pages (which would quickly establish the length of an inch, and the number of inches in a mile) missing. He didn't easily have a way to translate the scales expressed in them. *"Is a 'mile' two steps, three..?" So he might have presumed the Earth was much smaller, and the oceans much closer to being drained than they actually were.
Also, it was very difficult for the Scavs to move around without getting blasted by a drone, so 'stepping out' the distance between two places identified in the atlas would have been impractical.