Question

Why does an increase in altitude result in a decrease of thrust for a turbojet engine?

Answer 1

As we move up that is as altitude increase the density of air continuously decrease or air gets more rarified as a result aircraft speed increase.

and we know accelerating air produce Thurst. the acceleration is controlled by the difference between exhaust and incoming airspeed because exhaust air approx leaves with constant velocity. so as fast as aircraft the less acceleration is created on the intake air.

therefore as aircraft speed increase or in other word altitude increase thurst decrease.