Presumably, because if they increased their intelligence they would realize that the war is stupid and go home, which leaves fighting only those that did not. This starts to look like a rationalization, rather than serious reason, but then I always thought that Culture books are carefully constructed to retain as much as possible of “classic” science fiction (starships! lasers! aliens!) in the face of singularity.
Presumably, because if they increased their intelligence they would realize that the war is stupid and go home, which leaves fighting only those that did not. This starts to look like a rationalization, rather than serious reason, but then I always thought that Culture books are carefully constructed to retain as much as possible of “classic” science fiction (starships! lasers! aliens!) in the face of singularity.