Here's why:

* Mass is an intrinsic property: Mass is a fundamental property of matter, representing the amount of matter an object contains. It's like a fundamental building block, not something that can be altered by external forces.

* Force changes motion: Forces cause objects to accelerate (change their velocity) or deform (change their shape).

* Einstein's Relativity: While Einstein's theory of relativity tells us that mass and energy are equivalent, this doesn't mean force directly changes mass. It means that energy can be converted into mass and vice versa, but this requires massive energy transformations, not everyday forces.

Example: Imagine pushing a box across a floor. You are applying a force, and the box accelerates. The box's mass remains the same, even though its motion changes.

Important Note: There are scenarios in physics where the *perceived* mass of an object can change due to relativistic effects (like near the speed of light), but this isn't due to a force changing the object's inherent mass.