Because this wouldn't benefit voters in big cities. As a general rule, political units don't do things that are totally disconnected from the wellbeing of the p

Because this wouldn't benefit voters in big cities. As a general rule, political units don't do things that are totally disconnected from the wellbeing of the people who control the leadership. That doesn't mean cities always behave optimally in the interest of their constituents, but you won't see too much in the way of making arbitrary investments that are completely disconnected and actually against the interests of city dwellers.