I don't think density is expensive, on the contrary, density is cheap due to economies of scale. I think one reason might be food production and transport. Bef

I don't think density is expensive, on the contrary, density is cheap due to economies of scale. I think one reason might be food production and transport. Before the invention of refrigeration, fertiliser, mass transport, humans had to live rather close to food production, which stands in the way of density. Another reason might have been that really dense living required the ability to build high-rise houses, with concomitant requirements for being able to pump water high up, have elevators and so on. Reliable sewage systems and provision of fresh water was another problem. Some of the above became widely available only at the end of the 19th, beginning of the 20th century. At that time, the automobile encouraged sub-urbanisation. It may be only know that small, yet very dense cities are possible.