Why do planes fly as high as they do?
The air around the plane has nothing to do with its propulsion, just like rockets can travel in space. The plane creates force in one direction, and by Newton’s third law this results in it travelling in the opposite direction. Planes could travel higher up to the stratosphere, but by that altitude the planes would be in near-complete vacuums, and they would need to be specially designed (Eg, some jets). That said, some planes do go higher on purpose because then they can safely break the speed of sound (As opposed to suffering serious turbulence and sonic booms at lower altitudes).
