No body/tank is getting through the ironically named Demilitarized Zone dividing South and North Korea, without getting lit up with fast moving pieces of metal hitting with high kinetic energy.
Hence the tunnels.
North Korean government's long standing, highest priority goal is conquest of South Korea through any means, including military invasion.
A tunnel is useful; you can get past the landmines and other defenses. Causing an earthquake a few miles across the DMZ is far less useful.