The shortest distance between two points is a straight line. However, if you distort the surface on which the line is drawn, you distort the line.
One of the most popular maps of the Earth is the
Equirectangular projection which makes for easy coordinate systems because latitudes and longitudes are all equally spaced throughout the map.
Lines, when placed upon a polar disk which is then distorted into an equirectangular-type map, are altered in a very predictable way. Due to the nature of the polar disk, lines will always tend toward the center of the disk--the upper part of the map.
This can easily be seen in an image such as this:
Here we see three straight lines on a polar disk traced out onto an equirectangular map. Notice how they all curve upward toward the north pole (point A)? (Note: the central vertical line on the rectangular map is the equivalent of the international date line)
Now, if you take a gander at this map, you'll see that airlines don't always do this.
In fact, they often appear to do the exact opposite--bending southward instead of northward.
My question is this: if the Monopole Model of the Flat Earth is correct, why do commercial flights not take the obvious shorter route, opting instead for routes that are much much longer than they need to be?
Edit:
Here's an animation showing the real flight paths across a polar model:
To iterate the question, why do commercial airliners take these routes when the straight line routes (see the first animation) are vastly shorter?