Is it for cultural reasons?
If the Empire doesn’t automate its military for fear of rendering its sophonts obsolete, they could achieve both efficiency and individual excellence by having each soldier command a fractal armada of subsidiary AIs and war constructs as an independent general. The same principle can be applied to administration.
You want sophoncy all the way up and down the command chain, because sub-sophont AIs all the way to general level is just begging for a very brittle command chain with not a lot of decision-making power and/or info-synthesizing capability.
Even if you start with nothing but AI’s, the push will be to eventually employ fully sophont AI’s at various points of the command chain just so the war machine can take a hit.