Just watched the worst M*A*S*H episode

[Wounded Bear]

6. Given that the war only lasted for 3 years and the series ran 11...

I'm not surprised at some chronological inconsistancies.

The war started in Jun of 1950. By the spring of 1951 it had stabilized into trench warfare that lasted until the armistice in summer of 1953. The front lines moved a little, but nothing significant like WWII. The mountainous terrain lent itself to defense and static warfare.

https://en.wikipedia.org/wiki/Korean_War

M*A*S*H would have occurred in the later, non mobile part of the conflict. Thus the hospital camp never really moves much. Helps to keep production costs down. That gives a window of just over 2 years to contain the 11 seasons and 256 episodes.

https://en.wikipedia.org/wiki/M*A*S*H_(TV_series)