He may simply have been lucky in where he left her body.
I just got done reading an article by Ann Rule in which the victim was actually found in someone's backyard... two years after she disappeared. Granted, it was a large lot (one and a half acres) and the back property line was covered in blackberry brambles.
The victim's remains were found when the property owner decided to get rid of the blackberry brambles. In the Pacific northwest, the brambles are so thick that a dog can't get through them, let alone a human being.
My guess is that AGT left her body somewhere that is difficult or nearly impossible for a human to reach on foot. When I look at maps of the area, it seems to me that there are lots of places that fit that description.