Answer:
Why? Because of electron shells. Technically, they're not fully inert. They have very low reactivity potential, and can only be forced to become reactive with difficulty.
Explanation:
All chemical reactivity is made possible through the atom's electron arrangement. Electrons basically have shelves where they live, called "levels" or "shells". Each level is farther from the nucleus than the previous one. Atoms are most stable when their outer most shell (called the valence shell) is full. Atoms with an incomplete shell will react with other atoms, in an attempt to either fill out the outer shell, or to rid itself of it's valence electrons so that that previous level becomes a full valence level. If the valence shell ils already full, the atom will not be inclined to create compounds.
The first shell can hold up to two electrons. After the first two electrons, any additional electrons have to begin a new shell. The second shell can hold eight electrons before it becomes full. Helium is the first noble gas on the periodic table, having two protons and two electrons. Because helium's outer most shell is full, it does not react with other atoms.
By comparison, look at hydrogen and oxygen. Oxygen has eight electrons. The first two electrons occupy the first shell. The remaining six go to the second shell. This leaves the second shell with two empty spaces that can potentially be filled. Meanwhile, hydrogen has one electron, with it's valence shell having an empty space for one additional electron. Two hydrogen atoms give up their single electrons to an oxygen atom, so that all three end up with stable valence levels.
By the time an atom can fill out the second electron shell on it's own (10 total electrons) you end up with neon, the second noble gas.
