I don't disagree that cities (or, if we must, neighborhoods) are the right unit, but well... you give the example of Detroit and that's not a poster child for a well-functioning economic agent.
There is a problem of indirection, as chez17 alludes to. A "city" is too much: a place to do business, a place to live alone, a place to explore, a place to retire, and so on and so on. Some of these measures are so critical that failure can be crippling. Thus, a city might compete for the attention of international organizations like banks and the Olympic committee... but why? What's the actual value exchange happening there and how does it play out in the details? (I'm not asking "why should they", but "why are they".)
I meant that Detroit is an example of a city that did not meet the needs of the Market and so has effectively become bankrupt / collapsed while those people who could have moved to competing cities - I was trying to say that the competition between cities was working (although it's hardly a perfect market)
In answer to why - tax revenue is one less than compelling answer - my preference is to look back at the city states of Greece - they competed and fought for supremacy, for survival for riches - but mostly they fought because their city was their tribe and their home and the others were enemy. It's pretty human basic stuff I believe - nations are too big to get really worked up about but a city - it's the right sort of size.