Here's why:

* Inertia is the tendency of an object to resist changes in its state of motion.

* This means an object at rest will stay at rest, and an object in motion will stay in motion at a constant velocity, unless acted upon by an outside force.

So, a stationary object has inertia because it resists being set into motion. And an object in motion has inertia because it resists changes in its speed or direction.

Think of it like this:

* A bowling ball at rest is hard to push because it has inertia.

* A bowling ball rolling down a lane keeps rolling because it has inertia.

Inertia is a fundamental property of matter, not just a characteristic of objects at rest.