Aircraft taking long routes around the globe to get from Oceania to America

Are you sure that this is the case? What is the exact mi or km distance you are seeing for AKL to LAX, for example? If the game was actually doing this, you would find that no aircraft would be able to fly the route as there is currently no aircraft that could do this in one leg!

You may be referring to the small map that you see on the route screen. This is deceptive: while it will draw a line going in the wrong direction, the actual point to point distance should reflect the distance across the Pacific.

The map on the add route map is fixed. It shows from West to East stopping at the IDL I think. The line can't leave the map, so it shows the connection of the two points in a straight line from one side to the other. The route map tab or your airline details will show a more detailed route that does not have the constraints of a single, rigid map.