Here's why:

* Newton's First Law of Motion: This law states that 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 Velocity: If an object is moving at constant velocity, it means both its speed and direction are not changing.

* Unbalanced Force: An unbalanced force is a force that is not canceled out by an equal and opposite force. This is what causes a change in motion.

Since the object is moving at constant velocity, there is no change in motion. This means there cannot be any unbalanced forces acting on it. Therefore, the net force must be zero.