The fence could only be put in places where it was feasible to build. There are some "holes" in the border due to geographical difficulties. Also, remember google maps is slightly outdated. Google maps doesn't even show the fence near Yuma, AZ even though the entire area is pretty well fenced up.

 

Thats true. Plus the area I was in was very heavily patrolled by the border patrol. The fence is put up and maintaned in places where trafficking is frequent. Where I was, I only saw a few "runners" that will most likely be caught anyway.