Why are simple machines not 100% efficient?
Most simply, because nothing is 100% efficient.
Okay, got that out of the way... Now, practically, why are the simple machines not 100% efficient?
These fundamental truths come in many varieties, but in this specific case, The answer is friction...
Think about it:
a wheel and axle - friction of the rotating member against the non-rotating member (even the wheel against the ground). A pulley is just an extension of a wheel - friction between the rope and the pulley, more between the axle and the block.
An inclined plane - friction between the object on the plane and the plane itself. A screw is a special case of an inclined plane.
A wedge - again, friction (and this is very handy for door stoppers)
A lever - friction against the thing being moved, even with a fulcrum.
it is so obvious that our huge factory machines are made of simple machines in parts.. for the huge work that we need in our world cannot mach to simple machines. All we do is that complex that simple machines to work faster and in a improved version. this is why simple machines aren't 100% efficient compared to the world of haste we live in.