Here's why:

* Earth's Gravity: Gravity constantly pulls projectiles towards the center of the Earth. This pull causes a downward curve in the projectile's path.

* Inertia: Inertia keeps a projectile moving in a straight line at a constant speed *unless acted upon by an external force*. Gravity is that external force.

Therefore, projectiles travel in a curved path, not a straight one. The path is typically a parabola (or a close approximation of one).