Why can no machine be 100% efficient?
Because of gravity and friction
No machine is free from the effects of gravity, and even with wonderful lubrication, friction always exists. The energy a machine produces is always less than the energy put into it (energy input). Most machines transfer energy from one place or another, or transform one form of energy (e.g. chemical) into another (e.g. mechanical), but machines can`t create any form of energy. This tendency of systems to lose energy is called entropy.
Entropy in a closed system never goes down. Systems always tend toward a state of decreasing order unless more energy is provided into the system to counteract this tendency.
That is why 100% efficiency in machines shall not be possible.