In order to do so, we would have to shut down ALL air traffic , as well as all shipping too and build 40 foot high and down below, under ground electrified fences, topped with broken glass and backed by armed guards on a 24 hour basis. That just isn't ever going to happen.
Already been tried, ie; Berlin. Russkies did all that and more.
Guess what. Didn't work.
Now figure trying that with a 2500 mile border. Just the one. Like the man says, not gonna happen. Deal with it.