Here's why:

* Gravity acts vertically: Gravity only affects the vertical motion of a projectile. It pulls the object downwards, causing it to accelerate in the vertical direction.

* Horizontal motion is independent: The horizontal velocity of a projectile remains constant throughout its flight, assuming negligible air resistance. This is because there's no force acting horizontally on the projectile.

In summary: While gravity directly affects the vertical component of projectile motion, it doesn't influence the horizontal component, which remains constant.