Trump is convinced that failing to deliver on the Wall is what is really going to hurt his chances at reelection.
He has no idea that he could build a wal all the way around the whole country and it isnt going to move the needle for him. His fate is sealed.
The existing walls may as well not be there. Most are nothing more than 6 foot high sheet metal or just a pipe laying on the ground. They are starting with these areas, which are also the most critical areas because they can start immediately without the need for land acquisition...duh?