That and I don't know of anyone (outside of using them as particles) that has successfully created collision models with voxels in real time. The collision calculations of just a few of their scanned rocks and a ground plane would be plenty complex - a whole scene perhaps even a little insane given today's specs.

They might have to resort to polygonal collision models in the same way that polygonal games end up using low-poly collision models (with the same pitfalls such as moonwalking, blocked projectiles or mystery-bouncing).