Perhaps because they understand the reality of the situation better?
Look, almost everyone gets the situation with North Korea wrong. NK has exactly one deterrent from being invaded, and that is a ton of very boring, very conventional artillery that is within firing distance of Seoul.
In the case of NK being invaded, they would immediately start shelling Seoul and trying to reduce it to rubble.
If you start evacuating Seoul, NK will assume they're going to be invaded, and start shelling it.
In the improbable case that NK would start something, they would start by shelling Seoul.
All of these scenarios end with NK being reduced to rubble by the overwhelmingly superior force of South Korea + Japan + USA. That's the deterrent we have against them.
The nuclear missiles that NK is barking about is not a real threat. Everyone is watching NK 24/7,they cannot launch anything undetected. If something is detected, then South Korea, Japan, and the US will compete with each other about who can shoot the shit down first. And they will shoot the shit down. And after they've shot the shit down, they will invade NK to reduce it to rubble so as not to have to go through that again, at which point NK will start shelling Seoul, which sucks, but at that point it's an acceptable loss.
Every single scenario ends with NK reduced to rubble. They know this. They have zero reason to move the status quo. Instead, the real reason for them barking about their nuclear capabilities is to get more leverage in sanction discussions. And everyone goes through with this dog-and-pony show, because we can't openly state the exact capabilities of the anti-missile defense systems that are in place around NK.