Why can scientist assume elastic collisions as long as the temperature remains constant?

Because in elastic collisions there is no heat emission or absorption.

Explanation:

A collision is considered elastic when the total kinetic energy of the study system is conserved during the collision. Since the total kinetic energy is conserved, heat is not emitted or absorbed during the collision. Since the emission or absorption of heat is what produces changes in temperature, If the system remains at a constant temperature, there were only elastic collisions.