Why does the efficiency of a complex machine decreases as more simple machines are used ?

By the law of conservation of energy, the energy input to a system must equal the output. However, when there are more moving parts in a machine, like in a complex machine, the energy converted to non-useful energy, such as the production of heat energy via friction, is increased. Therefore, the more the moving parts, the greater the losses of energy due to increased friction. This is why complex machines are less efficient.  

well, you see. the answer to your question is quite simple. the thing that is big, powerful, and over 9,000 is your mom.