Air is thinner at higher altitudes, which mean they face less drag(air resistance.
Ever put your had out the window while driving and notice it has a tendency to be pushed back? Well imagine that but with an airplane, being slowed down by the air.
Technically planes could operate at similar speeds at lower altitudes, but the problem is its less fuel efficient due to the engines needing to work harder to maintain speed, so it costs more money, and it could create noise problems for those on the ground.
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