Why gases are often biatomic?

sure they do. when you re-enter the atmosphere the immense heat generated by drag and the catalytic properties of the metal surface break atmospheric oxygen and nitrogen within the hypersonic flow into atomic gases.

 

Out of the 11 elements among the first 103 which are gases at atmospheric pressure and room temperature, the majority (6) are monoatomic. These 6 never form molecules (with themselves).

The remaining 5 biatomic gases can all be dissociated into atoms - by application of large amounts of energy. On cooling, they recombine.

The reason hydrogen, fluorine and chlorine are biatomic is that they readily form one and only one bond. Once that is saturated, they do not readily form additional bonds to network into solids.

The reason nitrogen and oxygen are biatomic is that they being second period elements readily form multiple bonds, again saturating their bonds so as not to form networks.

Not all diatomic elements are gases. Bromine and iodine are not. Talking of first 103, not sure about astatine - the inconveniently short halflives (not over 8 hours) hamper crystal structure investigations....