So, the "drink your milkshake" level access isn't crazy.
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.
Hence the tunnels.
North Korean government's long standing, highest priority goal is conquest of South Korea through any means, including military invasion.
I'm disputing "earthquakes as offensive weapon" being useful. Not tunnels.