Here's why:

* Neglecting air resistance: In a simplified model, we assume there's no air resistance acting on the projectile horizontally. This means there's no force causing the projectile to speed up or slow down in the horizontal direction.

* Constant velocity: Since there's no horizontal force, the horizontal component of the projectile's velocity remains constant throughout its flight. This means it travels equal distances in equal intervals of time.

* Linear path: Due to the constant horizontal velocity, the projectile follows a straight line path in the horizontal direction.

Key Point: While the vertical component of a projectile's motion is affected by gravity (and thus accelerates), the horizontal component remains constant in the absence of air resistance.