[Alastor Mobius Toth]

They're not recycled corpses in the same manner as zombies, or Infested Terrans of Starcraft. They're essentially new organisms, that are based upon certain human structures, but their internal morphology (is that the term?) is quite different.

To elaborate, BETAs are a result of very, very advanced engineering - they don't have to be made from corpses, just basic carbon itself. The way I understood Superior, BETA collect human corpses, and then break them down into basic carbon elements, which are then used to form other BETA strains.

Another thing, is that we had seen BETAs on the lunar surface, which means they don't need to breathe (eliminating the common attack vector of most combat gasses), and can survive extraordinary amount of lethal cosmic radiation. This means that most WMDs are largely ineffective against BETAs, which is what Yuuko implies to have happened when USSR tried to use its huge WMD stockpile on them - it had virtually no effect, besides forcing Soviets further and further back, because they destroyed their own country.

So Biological and Chemical weapons (along with nukes), are simply impractical, especially since there's a big chance they they could also affect the dwindling human population.