Here's why:

* Chemical rocks are formed by the precipitation of minerals from a solution (like water). Rock salt forms when seawater evaporates, leaving behind concentrated salt deposits that eventually solidify.

* Organic rocks are formed from the remains of living organisms (like coal or limestone from shells). Rock salt doesn't originate from biological matter.

* Clastic rocks are made up of fragments of other rocks that have been cemented together (like sandstone or conglomerate). Rock salt forms through a different process.

Let me know if you have more questions!