Hydrogen gas exists as H2 rather then H. Why is this so?

Because hydrogen gas exists as a MOLECULE of 2 atoms

This is because hydrogen atoms each have one electron and require only one more for the very stable electron arragement of helium atoms. Hydrogen atoms obtain this arrangement by sharing electrons with each other. The charge of the resulting pair of electrons buzzing round the two hydrogen atoms locks the atoms in pairs to form hydrogen molecules (H2). This electron sharing with hydrogen atoms gives rise to a strong chemical bond called a hydrogen bond (a strong covalent bond).