There are at least two reasons why an economist might think this.
First, the economist would think of all the opportunity costs involved in making those cars. All of the materials and labor that went into the cars could have been used to make something else. The economist would realize that this is true because of scarcity.
Second, the economist might think about how the assembly line itself comes about because of scarcity. Labor is scarce and time is scarce and so both are valuable. Assembly lines are important because they save on both of these things. If we had all the labor and all the time in the world, there would be no need for labor-saving and time-saving devices like those on the assembly line.
Resource scarcity is an economic problem, and economic practice and principles attempt to satisfy human needs and wants using the scarce resources. However, the principles and practice have to contend with the fact that there is never enough of an economic resource to meet all human needs. Due to this fact, the economist is forced to view the opportunities and opportunity costs of undertaking certain ventures.
The hundreds of cars being assembled have taken up a significant amount of resources which may otherwise have been used in the development of other products. There is a need for transport, and it is this need that guides the use of the resources in making the cars instead of some other product that would also need similar materials. In that case, scarcity determines resource allocation, and this determines production priorities.
The economist may also view the assembly line as a factor of increased efficiency in attempts to satisfy the human need for transport. The advanced technology used to produce hundreds of cars is not enough to meet the human needs/demand for transport. This suggests that in spite of advanced technology, human needs may not be completely satisfied and thus, the assembly line is forced to endlessly produce hundreds of cars in an attempt to satisfy those needs. This further proves that there will never be enough cars with the required specifications to satisfy all conceivable needs and demands.
The assembly line, which is a technological advancement, attempts to stretch the already scarce resources (labor, time and materials) in order to continue/sustain production of the cars.