I used to wonder why do planes take a route along the land mass when flying long distance. There were theories floating around such as the availability of land in case of an emergency, the strength of radars, etc. The correct reason is because it is along the path of the great circle i.e the shortest distance between two points on a sphere. Planes take the great circle route unless there is a jet stream in the direction of travel.
The shortest path between Tokyo and San Francisco would be along the great circle route. However when flying from Tokyo to San Francisco, planes fly along the jet stream route and flight times are much shorter.