Gaia Vince runs through the reasons:
The denser the city, the more productive, efficient and powerful it becomes. The theoretical physicists, Luis Bettencourt and Geoffrey West calculated that if the population of a city is doubled, average wages go up by 15%, as do other measures of productivity, like patents per capita. Economic output of a city of 10 million people will be 15-20% higher than that of two cities of 5 million people. Incomes are on average five times higher in urbanised countries with a largely rural population. And at the same time, resource use and carbon emissions plummet by 15% for every doubling in density, because of more efficient use of infrastructure and better use of public transportation.
(By Evol in Farringdon, London, England via Street Art Utopia)