Simply stated, they're stupid. Or running into a door that is opened only when an objective is completed. I once found around 5 - 7 of my bot pals running around aimlessly in the room where the 3rd last objective is held. The runway can tend to slow down a bit, due to the fact that the plane seems to hold a high poly count. Unfortunately, this area is where most of the map's action takes place, hurting us more than helping.
Bots are simply too easy to play against.
The floorplan can be confusing, and the fact that it is quite easy to mistake a window for an entrance doesn't help this fact. Eventually you will learn. Just keep running around and eventually you'll find your way to where you'd like to be. There apparently is an airduct system in place around the airport, but I still cannot figure out how in the world to get in there. Defense is hard and easy at the same time.