Because our forces in Okinawa, Philippines, etc. are just kickin' back drinking Mai Tai's right? Let's not forget that our ground troops would probably not have anything to do with preventing a missile from landing on Guam (unless they have exceptional balance and can form a very tall human ladder). We have subs and aircraft carriers up the ass off the Korean peninsula. There's no need for us to be paying to house, feed and arm 28,000 tropps next to the Korean DMZ.
Last I heard, the South Koreans have a decent military of their own. Let them deal with it.
Created in 1948, following the division of Korea, the Republic of Korea Armed Forces is one of the largest standing armed forces in the world with a reported personnel strength of 3,539,000 in 2012 (639,000 active force and 2,900,000 regular reserve), and additional 300,000 paramilitary.
https://en.wikipedia.org/wiki/Republic_of_Korea_Armed_Forces