Ooh I got you there! You’re right theres a big difference

It has to do with how springy each material is - two perfectly stiff objects collide without deforming and it acts as described. But two fleshy spongy objects that would compress and deform and break upon impact - all of that deformation absorbs energy away from the kinetic energy imparted during the collision, and is damage to the bodies. The remaining energy not dissipated to damage, friction, sound, heat etc is retained between the bodies as motion

This compression also draws the impact out over a longer span of time - that’s critical because the force imparted during the collision is inversely proportionate to the time duration of the collision. When OP cites newtons law of opposite reactions, people typically think of this with respect to force but really its about momentum - the law actually states that force is the time derivative of momentum. Practically speaking this means when we analyze collisions we look at it as a step load imparted on an elastic body and the stiffness of that body determines the frequency at which the load is imparted, which informs us how much strain the object develops in response. In this case bodily bits would break as they strain so its not an elastic model, but this would only further dampen the impact. In fact some parts of the skeleton are intended to break first to prevent more serious damage. I believe the clavicle is one example but anatomy isn’t my strong suit

Of course the problem really is trying to apply any of this to DnD. It’s a damn game and bringing physics into it doesn’t make it any better, + most folks who do are pretty out of their depth