Hardware

South Korea does do it in some part for protection in my opinion. North Korea are a very unpleasant neighbour, allied with China, and SK depends on US support as a deterrent. The primary benefit is that it’s the heart of their economy, but I believe that defence is also a factor.

Being at a critical part of many crucial supply chains means that the US has a very strong interest in making sure SK keeps producing. Even if you look at advanced products mostly made in Taiwan - e.g. the nvidia H100s, the CoWoS packaging is done by SK Hynix.