The monsters loved concrete. No one knew why. There’s nothing inherently special about the stuff, but to the monsters, it was like candy.
They ate roads, bridges, and buildings.
They never intentionally killed a person. The monsters. They just ate the concrete, sometimes a bridge would fall and people would die. No one was ever attacked.
It turns out that human flesh is their poison. They’ll eat any kind of rock or metal, but a chomp of man flesh and they go belly up. Otherwise, they’re impossible to kill.
We’re running out of concrete, but we’ve got plenty of people.