The border fence between the U.S. and Mexico is pretty long. It also isn't very consistent, seeing as it stops and starts at times, leaving blank spaces of nothing but desert terrain. It is truly difficult to build such a sprawling and long man-made object. Besides, we never even knew that the border fence went out all the way into Arizona. That's actually pretty wild when you think about it.
Truth be told, and this may just be us slacking in the geography department, but we weren't too sure that Arizona even had a border with Mexico. Shows how much we know.