I don&#x27;t	think density is expensive, on the contrary, density is	cheap due to economies
of scale.<p>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.<p>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.<p>Reliable sewage	systems and provision of fresh water was	another
problem.<p>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.<p>It may be only know that small, yet very dense cities are possible.