Because not everything in the scene is static and the entire rest of the pipeline works with triangles.

I'm not convinced, because mixing ray-marched and triangulated geometry is quite doable. If you've already computed a ray intersection, generating a triangle and putting a burden on vertex processing makes little sense if the geometry is static. If you wanted it to be deformable, then yes you may want to use a vertex shader. But in this demo, none of the high resolution geometries are deformable AFAICT.