Here's why:

* Mechanical Advantage (MA): MA is the ratio of the output force (the force exerted by the machine) to the input force (the force you apply).

* Direction Change: When a machine only changes the direction of force, it doesn't actually amplify the force itself. The output force is the same as the input force.

Example:

Imagine a simple pulley system where you pull down on a rope to lift a weight. The pulley changes the direction of the force, but the effort you put in is equal to the weight you lift. In this case, the MA is 1.

Important Note: While such a machine doesn't increase the force, it can be very useful. Changing the direction of force can make a task easier to perform, even if it doesn't multiply the force.