The problem here is that one quickly realizes that often the minimum time route is not the same as the minimum fuel route. Computing minimum fuel routes requires the input of aircraft performance. Having worked on algorithms to do this back in the 80's for airlines I can assure you that it is not trivial.
An example: what if the minimum distance route requires a climb to 10000' over a mountain? Would you want to choose that route over a longer distance valley route?
Still, there are many cases where the minimum distance route (usually but not always the minimum time route as well) is good enough.