Where was the energy being mined from the water going?
It apparently wasn't going to Titan, since there was no settlement on Titan.
There was so much concern by the AI that they couldn't lose more drones or water processors as well as an apparent chronic shortage of spare parts that it doesn't seem like the energy was being harvested for producing stuff by the Tet/AI ship.
The only thing I can think of is that the AI almost lost the war with Earth and is damaged to the point that it needs the energy just to exist or for some long-term rebuilding/repair before it can regain the ability to produce large-scale materials for harvesters, drones, etc.