The border is 1954 miles. If you think all that can be patrolled, there wouldn't be need for a wall in the first place, in fact, reduction of BP is part of the goal.
As the terrain is, not all the border can have a wall built. Add to that landowners suing in court over their land being taken for it, and the growing negativity towards the wall resulting from eminent domain seizing so much land. One thing worse than illegals crossing your land is a wall crossing it and all that is no longer yours because the gov't took it.