Question

why efficiency of a machine always less than hundred percent

Answer 1

It is bcoz nothing in this world is perfect and in machines their is lot physics like friction in different parts,etc.

Answer 2

Well it's due to second law of thermodynamics. Which says entropy of the universe increases for any process. Even if we managed to reduce the resisting force crazily low ( like the other answer suggested), there is still a limit to how efficient an machine can be. Carnot engine is the most effective theoretical machine and it still isn't 100% efficient (you should learn more about that).

Basically second law of thermodynamics literally says "Dude you can't do it, because I say so"

Total badass.

You should learn more about entropy and thermodynamics. Khanacademy has really nice videos on them.