Seoul is withing artillery range of the DMZ
and reports are that the North has hundreds, perhaps thousands, of artillery pieces in hardened installations that could shell Seoul. They could lash out and kill thousands in no time. During the Korean War, the North overran Seoul more than once. Plus, the North is led by a bunch of lunatics. It isn't irrational to be scared of a lunatic with a gun. It's very rational to be scared of a bunch of lunatics with a lot of cannons aimed at you. Still, South Korea has about 600,000 men in their military and all the reports I've read say they're very professional (and mean).