Are you determined to only get the answer you want to hear ?
When a rock can no longer support life inside of it's self it will get covered in other more porous structures.
You have to think of your live rock like a sponge that takes things in, processes them and spits them back out.
If your rock is so solidified and hard that it can't do that then it is worthless.
It is found in the reefs yes, but it is old and dead and other organisms grow on top of it for lack of a better place to grow. The rock merely has a skin, not an integration of life.
Mr Neutron is 100% correct.