Here's why:
* Pearls are formed by living oysters or mollusks: Pearls are created when an irritant (like a grain of sand) enters the mollusk's shell. The mollusk then secretes layers of nacre (the same substance that makes up the shell) around the irritant to protect itself.
* The process requires a living organism: Once the pearl is removed from the oyster or mollusk, the biological process of nacre secretion stops. There's no longer a living organism depositing layers onto the pearl.
Think of it like harvesting an apple from a tree. The apple won't grow any further once it's picked because it's no longer attached to the living tree that provides it with the necessary nutrients and processes for growth.