Question

Why do inelastic collisions conserve momentum?

Answer 1

Ciao; even in inelastic collisions the conditions for conservation of momentum `vecp` apply:
The system is closed and isolated; so mass doesn't change AND the resultant of the external forces is zero!

Explanation:

In an inelastic collision the only thing that changes is kinetic energy!
Imagine a typical inelastic collision between two cars (that after get tangled together). The resultant of external forces is zero and no mass is added or subtracted exactly as in an elastic collision. The only difference is that some of the initial kinetic energy is used to make some deformation, heat up the two cars, noise…etc:

Spero che ti sia di aiuto!