Because we know that if we withdraw, SK will fall to the North in a matter of days?
By staying, we allow their SK's politicians to go around masquerading as holier-than-thou leftists, espousing a philosophy basically borrowed from John Lennon.
South Korea has twice the population and 50 times the economy of the North. They have far more sophisticated and functioning weapons. They can afford to buy the best of the best.
Why are we in the middle of this civil war?