Their scenario is that the ships are mostly going to be "fuel mules" to ferry propellant to the ship that is destined to go somewhere (i.e. Mars) - so if you wa

Their scenario is that the ships are mostly going to be "fuel mules" to ferry propellant to the ship that is destined to go somewhere (i.e. Mars) - so if you want an armada to travel to another planet, you need a much larger fleet of supply vehicles to prepare your armada. Hence the need to mass produce them.