3D shooters and Quake in particular are aiming games above. If you aim perfectly, you will always win against humans.
Adding randomness to aiming is the way to provide fair comparison to give any conclusion regarding true AI features.
The problem is that when it starts to play against humans the humans develop an anti-strategy after just a several matches. Feeding that back into AI doesn't immediately give enough data points for training.
But did you ever consider that an AI can just aim better than you period?
I'm totally blind. I could enter a marksmanship competition and demand that everybody be made to play by rules that would work for me, but I then wouldn't be able to say that I'm the best marksman if I win.