It's not so simple. Consider the basic equation. Mostly unpopulated, desolate, rugged terrain in the North where artillery can hide. Heavily populated metropolitan areas in the South where Northern artillery can hit. You would need to blanket a huge area of the North with devastating bombardment in order to remove all of the artillery positions. Meanwhile, every single shell fired from the North will likely result in civilian deaths in the South. And they can fire a lot of shells in a very short amount of time.

The only way to guarantee taking out all of the Northern artillery positions would be carpet bombing with nukes, which would result in lots of deaths in South Korea from fallout (not to mention all the other much more serious problems involved, such as establishing a precedent of preemptive nuclear attack).

In reality, the situation is even more complicated than this simplistic analysis. It's not so easy to take out an artillery position in a previously undetermined location, especially if that artillery position can be hidden in a cave/bunker on a moment's notice. More so if there are thousands of artillery positions spread over many square miles.

Even if the artillery in the North survives only a few hours or even a few minutes the sheer number of them and the population density of the regions of South Korea within artillery range would lead to catastrophic civilian deaths in the South.

The South Koreans have been militarily capable of stomping all over the North for decades, without American help. The fact that they haven't yet, despite numerous provocations from the North, is a testament to the huge losses that they would inevitably face if a hot war broke out again on the Korean peninsula.

Edit: Current estimates are that North Korea has the capability to fire around 10,000 rounds a minute into heavily populated regions near Seoul.