Here's why:
* Inertia is the tendency of an object to resist changes in its motion. This means that an object at rest wants to stay at rest, and an object in motion wants to stay in motion.
* Newton's First Law of Motion describes this concept: "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."
So, while there's no specific force "keeping" an object in motion, it's the absence of an unbalanced force that allows the object to continue moving.
