So it's quite an organic decision to select such a location, as also all the suppliers and customers are already located in this region.
Relocating to an entirely different region would just delay the process and actually risk to collapse the economy for quite mundane reasons.
Other Industries have their center of weight located more south, like Automotive and Energy.
One "mundane reason" to imagine the complexity:
People now working in semiconductor sector all live around Seoul. Apartments in Korea are not rented but predominantly purchased on loan.
Just announcing to pull a whole industry further away from Seoul would immediately drive up housing prices of the destination and collapse the housing prices in Seoul. So people selling to relocate could not pay off their loans with that money and could not acquire at the destination due to huge debt. Seoul real-estate market could collapse, and since most of it is owned on loan, it would at least immobilize the whole Seoul region (as people could not relocate without risking bancrupcy), at worst cause banks to no longer accept the apartments as sufficient security for the loan.