Utilizing technology, boots on the ground and strategic fencing is the best solution. The fact is resourceful people will simply cut though, climb over or tunnel under a fence that is left unattended.
There’s also treaties involved which could be a major reason for not fencing the entire border.
If one were serious about controlling the border, the fence would not be left unattended. Fencing has worked well many places including San Diego. Fences are just one part of a comprehensive border control plan. The border in Mexico is controlled by narco-terrorists, treaties are meaningless.