Let us assume that we were to do exactly what you suggest and declare the entire border a port of entry. Upon crossing the border, they would then be in a customs controlled area, and they would then still have the obligation to present to an officer, both for customs and, if they are foreign nationals, for immigration purposes. You would have to be able to offer those services all along that border of 8,000 plus kilometres.
On May 3rd, 2018. See this statement in context.