Answer:
Land scarcity and high land values - CBDs are located in the most prime and valuable areas of a city, where land is in limited supply. Building upwards allows businesses to maximize the use of this scarce and expensive land.
Density and concentration of economic activity - CBDs are hubs of commercial, financial, and administrative activity. Multi-storey buildings allow for the concentration of a large number of businesses, offices, and workers in a relatively small geographic area.
Efficiency and agglomeration economies - Clustering businesses and workers in a dense, vertical configuration creates economies of scale and agglomeration benefits like easier collaboration, access to shared infrastructure and amenities, and spillover of knowledge and innovation.
Transportation and accessibility - Locating many businesses and workers in a central, transit-accessible area reduces the need for lengthy commutes and allows for more efficient transportation networks like subways, trains, and buses.
Prestige and visibility - Tall, prominent buildings in a CBD can serve as landmarks and symbols of a city's economic power and dynamism, attracting further investment and activity to the area.
So in summary, the multi-storey nature of CBD buildings primarily arises from the need to intensively utilize scarce and valuable urban land in order to support a high concentration of commercial activity and economic productivity in the heart of the city.
