Because alien species mate and reproduce, growing exponentially. Hidding a bomb in a building is a much milder menace that causes a damage once.
This would be like using a bomb able to make and disseminate more bombs, each one able to make and disseminate more bombs before exploding. And the iteration runs forever basically.
This would be like using a bomb able to make and disseminate more bombs, each one able to make and disseminate more bombs before exploding. And the iteration runs forever basically.