Here's why:

* Momentum is a vector quantity: It has both magnitude (how much) and direction.

* Conservation of momentum: The total momentum of a closed system (one where no external forces act) remains constant. This means the total momentum *before* the collision is equal to the total momentum *after* the collision.

In an inelastic collision:

* Some kinetic energy is lost, often converted into heat, sound, or deformation of the objects involved.

* The objects may stick together or deform significantly after the collision.

* However, the total momentum of the system remains conserved.

Example:

Imagine a car hitting a stationary object. The car might crumple, and some kinetic energy is lost as heat and sound. However, the total momentum of the car and the object before the collision (just the car's momentum) is equal to the total momentum of the car and object after the collision (the combined momentum of the car and object).

Key point: While energy might not be conserved in an inelastic collision, momentum is always conserved.