Here's why:

* Newton's First Law of Motion (Inertia): An object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

* Constant Speed: If an object is moving at a constant speed, it means its velocity is not changing. Velocity has both speed and direction.

* Force and Acceleration: A force causes an object to accelerate, meaning a change in its velocity.

* Net Force: The net force is the sum of all forces acting on an object. If the net force is zero, there is no acceleration.

Therefore, if an object is traveling at a constant speed, the forces acting on it must be balanced, resulting in a net force of zero.