Remember the first WTC bombing? They were able to identify the perpetrators because they went back for their deposit on the rental truck.
Although Atta and his fellow ringleaders appear to be well-trained, they no doubt have to rely upon a network of support assets who are not trained to the same level and do not exhibit the same level of tradecraft or discipline.
Wasn't it Atta that stated he only wanted to learn how to fly a plane,not land?? I don't think they are all that smart.