The Korean DMZ is 160 miles wide. The U.S. Mexican border is 1,969 miles.
I've had personal experience with mine fields as a result of my Navy tour at Guantanamo and minefields extract a high cost.
First of all, minefields are Equal Opportunity and blow up anything that wanders into them from drunk American sailors who don't see the minefield signs at night (a tragic incident before my tour when sailors off a visiting warship accidentally wandered into the mine fields) to any stray animals (deer in Cuba or cattle in Texas).
Secondly, mine fields are high maintenance. They need actual American servicemen to go in there and keep it serviceable. I had a several Minefield Maintenance Marines come into my Sick Call, their nerves shot, begging me to give them a medical excuse to change duties.
Thirdly, Minefield Maintenance sh#t happens. One Marine on Minefield Maintenence was flow into my ER, legs blown off below the groin by a Bouncing Betty that he accidentally activated. No, he did not make it.
Minefields may sound really cool in theory but they are extremely nasty things in real life.
Ain't gonna happen on the U.S. border.
Marine Barracks Minefield Maintenance personnel unload deactivated anti-tank and anti-personnel land mines for destruction at a demolition site on Naval Station Guantanamo Bay, Cuba, March 18, 1997