A rock in space still has photons hitting it from various sources, so it still interacts with its environment, which prevents any macroscopic superposition from happening.

If you eliminate the sources of photons (i.e. suppose the rock were in an empty universe), then you can't see the rock anymore, so there's no problem there either.

A rock in space still has photons hitting it from various sources, so it still interacts with its environment, which prevents any macroscopic superposition from happening.

If you eliminate the sources of photons (i.e. suppose the rock were in an empty universe, and at zero temperature), then you can't see the rock anymore, so there's no problem there either.