Because, in an impulse-constraint system, all bounces happen instantaneously. An impulse is an infinite force applied over zero time but with a finite energy transfer product. This results in an infinite acceleration over zero time, with a finite change in the velocity vector. That's not how the macroscopic universe works. It doesn't properly model a combined impact/skin compress/slide/skin expand/bounce situation for heavy objects, which is not instantaneous.. This shows. It's why objects banging around in video games all seem to be too light.

A better model is to use compressible "skin" over incompressible "bones", which is what Falling Bodies did.