Interestingly it might just have to do with the age of the roads. When I was living in Baltimore in the late 80s my fellow Californians complained how bad the roads were in Maryland compared to LA. Now 30 years later the roads in California are bad too. IIUC much of Korea's roads are new-ish so it's possible in 30 years they'll be in the same state. .... or not of course. Maybe Korea is just better at maintaining roads.