Kinetic energy is lost in inelastic collisions due to the very definition of inelasticity, which involves deformation and energy dissipation. In contrast, elastic collisions conserve kinetic energy, allowing the objects to rebound without permanent deformation. The energy that is lost during an inelastic collision typically transforms into heat energy within the colliding bodies. Understanding the distinction between elastic and inelastic collisions is crucial for grasping the principles of energy conservation in physics. This discussion highlights the fundamental differences in energy behavior during these types of collisions.