Life in a big city is no cure for poverty. Urbanization accelerated urban growth by adding more territories to cities. Lack of jobs pushed the rural population to cities where they have more chances to find employment. Most of these people are poor and have few chances to get out of poverty even after they find a job. This cycle blocks poverty inside the city and does not let communities develop.
Poverty in megalopolises is definitely caused by poverty in its rural suburbs. Poor people will anyway remain poor as they move to cities, and labor migrants comprise a great part of the urban population. The lack of education does not let most rural migrants qualify for a highly-paid job, which makes them marginalized among the all-time urban population. Many people lack social security in big cities, and are even more likely to trap into poverty moving there. In fact, not all of the migrants can find even a low-paid job moving to a new place.