* Full Valence Shells: Noble gases have a full outer shell of electrons, making them exceptionally stable. They don't need to gain any more electrons to achieve a stable configuration.

* High Ionization Energies: It takes a significant amount of energy to remove an electron from a noble gas atom. This means adding an electron (which would mean the atom becomes an anion) would be highly unfavorable.

Technically, noble gases do have a slightly negative electron affinity, meaning they do have a very slight tendency to gain an electron. However, this tendency is so weak that it's often considered negligible.

In summary: While noble gases technically have a very small electron affinity, their exceptionally stable electron configurations make them essentially unreactive and unable to readily gain electrons.