Why is the Motor-City in Detroit

[Don K]

But I really don't know where any of those ugly AMCs were made...

Well, the AMCs came off the line in Kenosha, WI.

Back to the original topic, I suppose the main reasons Detroit became the Motor City are the introduction of the Model T by Henry I and the formation of GM by William Durant, both in 1908.

I'm not really a believer in geographic determinism, because it gives no way to explain why Detroit became the Motor City and not Chicago, Cleveland, Milwaukee, or Kenosha. I'd say it was just happenstance that Henry was born in Greenfield Township and Durant was in control of Buick and was able to leverage that into the creation of GM. Once those two companies were dominant in the industry, then Detroit was by default the Silicon Valley of the era, where suppliers had to be located.