Because the air is thinner there, so going fast takes less energy.
Imagine if you had to compete in a swimming race, one pool is regular pool water, the other is full of jelly. Which would you be quickest in? The water, for sure.
Planes flying at higher altitude are much more efficient than low altitude. They use less fuel for the same speed and distance travelled.
Since we all like paying as little as possible to travel, it’s in our interest.s
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