A system can never be 100% efficient. The reason behind this is because of heat and sound energy are lost in some amount in every system. No matter they are waste or not. So a system doesn't exist, with 100% efficiency.
In this way, why is the efficiency of a machine not 100%?
Mechanical efficiency. Mechanical efficiency measures the effectiveness of a machine in transforming the energy and power that is input to the device into an output force and movement. Real devices will have efficiency less than 100% because rigid and friction-less systems do not exist.
Is it possible to have a 100 efficient machine?
A simple machine, such as a lever, pulley, or gear train, is "ideal" if the power input is equal to the power output of the device, which means there are no losses. In this case, the mechanical efficiency is 100%.