Here's why:

* It's synthetic: Ununbium is a synthetic element, meaning it doesn't exist naturally on Earth. It is created in laboratories through nuclear reactions.

* Highly unstable: Ununbium is extremely radioactive and has a very short half-life. This means its atoms decay rapidly, making it impossible to find in natural deposits.

* No stable isotopes: All isotopes of ununbium are unstable and decay quickly. This further contributes to its non-existence in nature.

So, while you might see the element listed on the periodic table, you won't find it naturally occurring anywhere on Earth.