Why planes fly high during cruise?

Well, some of you have been asking. Why do planes fly high during cruise? Here is why ↓

Planes fly high during cruise because the air is thinner at a high altitude, which makes the engine more efficient but why don't planes fly higher? Planes can't fly higher because the air will be too thin so the plane can't generate any lift and result in a stall (with exceptions of military planes and the concorde)

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Written by: Sohail Sawlani