Why not? What do you understand "aiki" to mean? Why can't one manifest it while attacking?
Well, I'm not saying that uke can't exhibit elements of 'aiki'. In fact they should. Uke should maintain balance, and blend with the technique once it has begun, which can lead to kaeshi-waza etc etc etc. However, when it comes down to it, they are still trying to hit nage in the head.
As for what I understand aiki to mean, I suppose a large part of it is harmonising forces. Whether those forces are within, or come from an attacker or are metaphorical or whatever is up to the interpreter. In any case, I am yet to be convinced that hitting someone in the head is particularly aiki. I am, however, willing to be convinced. Could you explain how you see it?