Here are two instances during which the Rigid Dynamics solver tries to mesh the parts:
1. When you are doing rigid-flexible body dynamics; only the parts that are marked as flexible are meshed.
2. When you have any contacts defined in your model; the contact algorithm uses the mesh nodes as contact detection points so it creates a surface mesh on the faces that are included under contact/target scoping.
If you don’t have either of these things defined in your model, the rigid dynamics solver should not generate any mesh on the rigid bodies.