Here's why:

* Work is defined as the force applied in the direction of displacement.

* If the force is perpendicular to the displacement, the work done is zero.

In this case, the vertical force is perpendicular to the horizontal displacement. Think of it like pushing a box across the floor. If you push straight down on the box, you might apply a force, but you're not making the box move horizontally. You're only pushing it against the ground.

Important Note: Even though the vertical force doesn't do work, it can still have an impact on the object. For example, it could contribute to a change in the object's vertical velocity or create friction, which could then impact the horizontal motion.