I hate whhen people attribute map problems to the Mercator projection without properly learning about what is actually wrong with it.

1. This is not a Mercator projection map. The northern side of the US is straight and horizontal on Mercartor maps.
2. "Straight lines" on a globe (great circles) on the Northern hemisphere bend in this ⏜ way, not this ⏝ on a Mercartor map.
3. Flattening a sphere without compromises is impossible. Most other projections are either unusable in many local contexts or don't span the globe. Mercator (specifically the WGS-84 implementation) works very well unless your journey goes over ±84° latitude, which is OK for most cases. Every other projection will stretch, skew or cut local areas in major ways and EVERY ONE WILL SHOW (almost every instance of) THE SHORTEST (great circle) PATH AS NOT STRAIGHT. And the last, all-caps point only manifests for long journeys. If you can travel long distances in a straight line, you are piloting a ship or aircraft, and you can afford a computer to plot the great circle curve.

Actual problems with Mercator:

1. It needs to be infinitely tall to cover poles. This map shows how tall a version that gets to .1 mm from the South Pole is. Therefore, polar researchers and the military need to get another app for navigation (along with hardware for whatever the substitute for GPS is). Like every map projection, it distorts "long" straight lines (look at the Amundsen-Scott station building) but what's important, angles and bearings are preserved (the building still has 4 right angles).
2. It significantly distorts sizes at different latitudes so it's not recommended for showing the entire world, especially for a child's first impression. There are worse picks, such as Gall-Peters (which preserves sizes but stretches shapes horribly unless you are close to the very conveniently picked 45° latitude where most of the Europe lives, nullifying the notion that the map combats imperiallism) or Dymaxion (which I love but cutting oceans and the equator into incongruent pieces strewn all over the page does not help children grasp basic geography).

nope. This is the straight line path on a Mercator map

"Sadly" not really. As this is the northern hemisphere the "shorter" path would be an arc opening downwards. As others have commented, there are other reasons for this route.

If that is the case, then op is wrong, that's not how mercator works (the opposite actually).
(I'm glad this isn't Reddit or I would be forced by custom to explain in detail the sexy times I had with their mother.)

But aviation logistic is complex (duh), there are so many things.

Among which I now realise I previously forgot to mention various weather phenomena (that can make you save fuel).