An Analogy ... of Sorts

A group of us were discussing the incredible movement of people from rural areas to urban areas in Mexico. Friends who live in Mexico City and in Tijuana told us recently that up to 1,000 people move to those two cities each day.
I live in a town of 6,000 people. Imagine if 1,000 people moved to my town each day. In one week, we would double our town. Add a few more weeks and our town would be quadrupled.
Here's what would happen:
Our supply of clean, safe drinking water could not keep up with that kind of influx.
Our sewage system could't beginto cope with that influx.
We don't have near enough homes for all those people so they would start doubling up, living in any abandoned buildings they could find, and when the growth continued, they would start squatting on farmer's land near the town so they wouldn't be too far from any potential jobs.
In short order, we would have the beginnings of a slum in the rural heartland of N'W Iowa.
It's a simple analogy, but extrapolate that to what's happening in Manila, Kinshasha, Bucharest, Mumbai and so many other places. When you look at it this way, it becomes understandable why so many places have informal housing, lack of sanitation and security, just lack of planning. How does one plan effectively for an influx of 1,000 people a day?
There's more to it than this little story illustrates, but it's a start on the road to understanding.