The issue, as always, is cost/benefit. N. Korea has always tried to walk the line between wanting to seem strong enough that you can't just toss the leaders out and not so threatening that you actually worry they might mess you or your allies up.

It's not worth it to the U.S. or anyone else to invade, lose 30K-100K soldiers and then have to rebuild the country, and it never has been. The nuclear arms race serves partially as a deterrent against possible invasion, and partially an attempt to get people to the negotiating table for some resources by scaring Japan and N. Korea enough that they want to deal with this.

But if they had the capability, they still don't have the desire to use it as a first strike since that would give every country around + the U.S. a reason to bomb them to the ground.