Then why in the game even conventional weapons can kill them?
Their power was in unrelenting size, numbers and mass, not really indestructibility.
their power also comes in the form of infectibility one spore, one infection form is all it takes. They can infect people very easily and as it infects it becomes more intelligent and will use weapons of the enemy to spread and make more of itself.
if zombies began to use firearms it would take firearms to kill them. the flood in its prime could use fleets of star ships and corrupt AI so it took alot to fight them.