Isn't the DMZ kind of an impediment to the South Korean Army marching anywhere? It's wide. It's riddled with land mines. Razor wire topped fences and guard shacks completely block the north from the south. North Koreans who want to go to South Korea have to escape through China.
The farther an obstacle is from the South Korean border, the more of an obstacle it will be to the South Korean armed forces. And North Korea really is one **** mountain after another.