You're right. Nobody is going to build infrastructure like this with a 'build it an they'll come' type of attitude. The authorities in China reserve the right to plan whatever they want, and the distribution of populations by any means they think is best is one of the basic rights totalitarian regimes give themselves. There's definitely a plan afoot here, but the strategy behind it is a mystery to me. During the Stalin/Khrushchev years large cities were suddenly established in the middle of nowhere for strategic, industrial, agricultural, etc. reasons which were known only to big shots in the party. Soviet planners didn't advertise their plans to the world, and the chicoms are similarly inscrutable.
You are forgetting that China has a severe problem with peasant labor from the countryside that has moved to the cities to find work. The only work those people can do is menial construction labor. What China has done is create jobs for those people by building things, whether they are needed or not. Remember there has been rampant real estate speculation there.
There are more than 900 million uneducated peasants in China. Many of them are unhappy with their lot in life after they realized they are not getting their share of the growth pie. China has to keep those idle hands busy or suffer rebellion.