Remember less than 1/2 the cells in your body actually contain your DNA. Yet signatures still work. Biological viruses are light years ahead of computer viruses. Picture a world where there where more computer virus strains than computers, they mutated millions of times a second, ignored firewalls and could attack computers though the electrical system. Further 1/4th of every nations GDP went to anti virus systems and had been for the last billion years.

PS: Virus writers have become far more tame but old school virus's would rewrite your BIOS.