Here's why:

* Full outer shell: Noble gases have a full outer shell of electrons, making them very stable. They don't need to gain or lose electrons to achieve a stable configuration.

* High ionization energy: It takes a lot of energy to remove an electron from a noble gas atom because of their stable electron configuration.

* Low electron affinity: They don't readily gain electrons either, as their outer shell is already full.

This low reactivity is why they were originally called "inert gases." However, it has been discovered that some noble gases can form compounds under specific conditions, though these reactions are still relatively rare.