The reason for this is down to simple mathematics and physics. The circumference of the Earth is a lot further around the equator than it is at higher or lower latitudes towards the poles of the earth, such as the spherical shape of our planet. Flying around the smaller circumference of the Earth is called the “Great Circle Route” and also very noticeable for flights from the U.S. to Asia that will fly far above Alaska and Siberia rather than what would appear to be a straight line.
If you think about drawing a line around a globe in the middle where it is widest, versus towards the North or South poles, the differentiation of distance begins to become apparent, and it not only saves a huge amount of time but also fuel. Because the Earth revolves on its axis, this forces the equator to “bulge out” and be wider. Despite what some theories suggest, the Earth is not flat, and so curvature becomes an incredibly important factor in routes aircraft take. Similar to the Earth itself, aircraft, therefore, take flight routes that also appear to be a curved line, tracing the Earth’s shape.