Air defense radars, missile launchers and even Seoul besides northern outskirts aren't generally in a range of tube artillery, which for the 152mm soviet types

Air defense radars, missile launchers and even Seoul besides northern outskirts aren't generally in a range of tube artillery, which for the 152mm soviet types utilized by NK is around 20km. And you're not putting your guns literally at the frontline - you need them more back so they don't get counterbatteried (or just bombed, which would be more likely in case of US involvement) instantly.