I'd imagine it's for the same reason we don't drop nuclear warheads on everyone we come into conflict with even thought it's a pretty efficient way to wipe enem

I'd imagine it's for the same reason we don't drop nuclear warheads on everyone we come into conflict with even thought it's a pretty efficient way to wipe enemies out. Infectious diseases don't discriminate, they can mutate and spreading them means your enemy can get a sample to weaponize.