The value of an aircraft depreciates much faster than that of engines.
The most valuable and expensive single component on an aircraft is its engine. Add two or four engines to an airplane, and the combined value becomes immense. Of course, engines are the essence of an aircraft, providing the required thrust to achieve the designed mission. The lifetime of an airframe in comparison to the engine is different for several reasons. This article explains the proportionality of the life of the aircraft in comparison to that of the engines.