Jet engines in order to operate require a mix of the fuel-air mixture known as stoichiometric ratio. What this means broadly is that the ratio of fuel to air molecules have to be precise in order to reach maximum operational efficiency. As the altitude increases the air becomes thin.
Or in other words, the density of air would decrease resulting in a lower number of air molecules entering into the system. As a consequence, the number of fuel molecules entering the engine will also have to be lowered to match the number of air molecules. Ultimately this results in a lower amount of fuel be burned to be operationally effective.
Hence it can be concluded that at higher altitudes the aircraft becomes more fuel efficient hence more economical too. Now, this is one reason, secondly, as the aircraft reaches the higher altitudes the air friction on the aircraft also decreases hence enhancing the overall fuel economy of the aircraft.
Aircraft apart from the above-mentioned reason, also fly high in the sky as a safety precaution. You see when you fly too high then the chances of hitting an obstacle are minimal. The aircraft frame is actually very sensitive to hits at high speeds even a bird hitting an aircraft can be disastrous. Hence flying high reduces such risks.