Why is it extremely unlikely to find biological compounds containing atoms of neon or argon?
Basically, neon and argon are noble gases and their atoms tends nor to react, neither to bond with atoms of other elements due to a low chemical reactivity. In living biological systems, thousands of chemical reactions take place, so you don't need high quantity of atoms with such low reactivity here. Biological compounds containing atoms of neon or argon seem to be quite useless in terms of biological systems. Because of low reactivity they cannot be effectively involved into major biochemical cycles. Of course, it is not at all easy for living organisms to obtain neon and argon from the environment.