Why aeroplanes do not fly high in the stratosphere?

Answer 1

Commercial airlines often cruise in the stratosphere, albeit at its lower reaches. However, airplanes fly due to the lift created by air flowing over/under their wings. An airplane's engines provide thrust, which move its wings through the air (i.e., increases the flow of air over/under the wings). Once the lift thus created by this airflow exceeds the airplane's weight, the airplane climbs into the air.

At higher altitudes, there is less air. So, more thrust is required in order for the wing to create enough airflow and lift to keep the airplane flying. A practical altitude limit is reached when the airplane's engines cannot provide enough thrust and/or its wings cannot produce enough lift in order to offset the airplane's weight.